How Many Units to Buy on an Online Arbitrage Deal

1. The Question Every OA Seller Gets Wrong

You find a deal. ROI is solid, BSR is dropping, profit per unit after fees checks out. Then you stare at the buy screen and freeze: how many units do I actually order?

Buy too few and you miss profit. Buy too many and your capital sits locked in an Amazon warehouse for 90 days while storage fees eat your margin. This decision trips up more new sellers than almost anything else in the business.

Four factors determine the right buy quantity. I run the same check on every deal at $100K+/month. If you're still building your OA foundation, start with the full online arbitrage guide first, then come back here.

2. Factor 1 - How Fast Is the ASIN Actually Selling?

Every unit quantity decision starts with sales velocity: how many units is this ASIN moving per month across all sellers?

BSR alone doesn't answer that question. A rank of 5,000 in grocery means something different from a rank of 5,000 in industrial tools. Look at Keepa's historical rank chart and count the drops to get a units-per-month estimate.

My minimum before I pursue a deal: 30-60 total units sold per month across all sellers. If the whole ASIN moves 8 units a month, buying 4 units clears at a crawl. At that velocity, find a faster product.

My Keepa graphs tutorial covers how to read sales velocity from Keepa. Read it before you source anything.

3. Factor 2 - How Many Sellers Are Splitting the Buy Box?

Most beginners look at total ASIN sales and assume that's what they'll sell. Those are two different numbers.

If an ASIN moves 60 units a month with 15 active FBA sellers at the same price, your realistic share is 4 units a month. Divide total monthly sales by active FBA sellers to get your number. Pull offer count from SellerAmp or Keepa and run that math before you commit a dollar.

High competition on a slow seller is a pass. High competition on a fast seller can work, but run the actual numbers first.

4. Factor 3 - How Many Days of Supply Do You Want to Hold?

This factor depends on your capital and comfort level, not the ASIN. From a video I shot on this topic:

"There's a couple of elements, a couple of things in play, that are external to you. But there's one thing that really depends on you: how long do you want your inventory to last? How many days of supply do you want to have? And it all depends on how much capital you have and what you are comfortable with."

4 EASY Ways to Know How Many Units to Buy - YouTube

When I was starting with under $5K, I targeted 30-day supply on most deals. That kept cash cycling fast and kept buying options open. Now I'll go 60-90 days on proven ASINs because my capital base supports the longer hold.

New sellers should default to 30-day supply targets until capital can absorb a slower turn. Moving capital fast through multiple deals beats parking it all in one.

5. Factor 4 - Your Expected Buy Box Share Percentage

After you know total monthly velocity, you still need a realistic estimate of your buy box share at the current competition level.

With 1-3 FBA sellers, you can capture 30-50% of sales. With 8-12 sellers, plan on 10-15%. With 15+ sellers at the same price, expect single-digit percentages.

A product moving 120 units a month sounds great until you count 20 sellers on the listing. Your realistic share is 6 units a month. Buying 15 units means sitting on 9 units for two extra months.

Check Keepa's buy box ownership chart to see the historical rotation. Read it before you estimate your share as a new seller entering the listing.

6. How the Four Factors Work Together

I teach this formula to students in The Scaling Society:

"For everything that you find, you will be able to decide how long you want to take the lead. Try to estimate how many items that specific ASIN sells, and from there try to see how much of the buy box you can get. What percentage makes sense? And then once again, you put those two things in relation to how long you want to hold."

4 EASY Ways to Know How Many Units to Buy - YouTube

The formula:

Units to buy = Monthly ASIN sales x Your buy box % x (Days of supply / 30)

Example: ASIN sells 90 units/month. You expect 25% buy box share. You want 45 days of supply.
90 x 0.25 x 1.5 = 33.75. Round down to 30 units. That's your buy.

Run this on every deal. The inputs change by product; the formula doesn't.

7. How Your ROI Target Changes How Aggressively You Buy

Your ROI minimum determines both whether to take a deal and how many units to buy on it.

At 15% ROI, keep the position small. A 10% price drop puts you at breakeven. Take a test buy and move on to better deals.

At 40%+ ROI with solid velocity and low competition, buy 60-90 days of supply. Even if it takes 60 days to fully clear, the return justifies the hold. The specific ROI thresholds I use are in my post on OA ROI minimums.

Higher ROI means more units and a longer acceptable hold. Under 20% ROI means small positions and fast turns. Stack this on top of the four-factor formula.

8. Q4 Changes the Formula Completely

October through December, the numbers change. Velocity doubles or triples on many ASINs. Buy box competition thins out on items with spiking demand. Prices rise and hold. Products sell at the highest velocity of the year.

In Q4, I buy 2x to 3x my normal days-of-supply target on proven Q4 performers. If I'd normally buy 20 units, I'm looking at 50-60 heading into November because I have the velocity data from Q4 last year.

Miss the window and you're waiting until next year. I cover the Q4 inventory strategy in my Q4 inventory planning guide. Get inventory into Amazon warehouses by October 15th to avoid holiday peak fulfillment fees.

Q4 puts more stress on cash than any other quarter. To watch how I plan actual Q4 buys in real time, grab a seat at the free Thursday training.

9. Capital Constraints Are Real - Work Within Them

If you have $2,000 in buying power, putting $1,600 into one ASIN at 80 units is a mistake. That's 80% of your capital in one bet. If the price crashes or Amazon restricts the listing, you're stuck for the month.

My rule for newer sellers: put no more than 20-25% of available capital into a single ASIN. With $2,000, that's $400-500 per product. Spread that across 5-8 deals and one bad position doesn't wreck the month.

As capital grows, this limit matters less. 20% of $20,000 is still $4,000 per deal, which is real buying power. Early on, spreading buys across 5-8 products is your primary risk management tool.

If capital is the bottleneck, check my post on FBA inventory financing options. There are legitimate ways to expand your buying power without bad debt.

10. The Test Buy Rule for Every New ASIN

No matter what the data shows, start with a test buy on any ASIN you've never sold. I do 3-5 units on unproven products, every time.

Data shows what should happen. Reality doesn't always match. Maybe the buy box is more contested than Keepa shows because five new sellers jumped in last week. Maybe there's a buried IP complaint that only surfaces after you're in. Maybe the item takes 3 weeks to go live and the price has already moved.

Test, confirm it sells at the expected velocity, then scale up on the restock. Test buys keep you out of the cash flow trap. I've covered how that trap works in my post on the FBA cash flow trap.

Once you've sold an ASIN two or three times at expected velocity, scale your unit quantity up.

11. Three Unit-Quantity Mistakes That Cost Sellers the Most

Three mistakes, in order of damage, from watching hundreds of sellers inside The Scaling Society:

1. Buying based on total ASIN velocity, not your share. BSR confirms a product has buyers. It doesn't split that demand across the 15 sellers competing for it. Divide total velocity by active seller count before you decide on quantity.
2. Buying 90-day supply with $3K in capital. That ties up your buying power in one product for three months while you miss other deals. Turn speed matters more than unit count when capital is limited.
3. Ignoring seasonality on the Keepa chart. A toy with great velocity in December will sit for 11 months if you buy it in February. Pull the two-year Keepa chart and check before buying anything seasonal.

For the full list of OA pitfalls, read my post on online arbitrage mistakes beginners make.

12. A Quick Decision Matrix for Every Deal

When I'm moving fast through a buy list, I use this matrix:

• 1-3 FBA sellers + 35%+ ROI + Strong velocity (60+ units/month): Buy 60-90 days of your estimated share.
• 4-8 sellers + 20-35% ROI + Moderate velocity (30-60 units/month): Buy 30-45 days of your estimated share.
• 9+ sellers + Under 20% ROI + Inconsistent velocity: Test buy only. 3-10 units max.
• Any ASIN you've never sold before: 3-5 units, regardless of how strong the data looks.

Seasonality, capital, and confidence in velocity data all change these numbers. This matrix keeps you out of bad positions on 90% of deals. Use it as a starting point and adjust from there.

13. Next Steps

Five posts to build out the rest of your OA operation:

• The Complete Online Arbitrage Guide for Amazon Sellers
• How to Read Keepa Graphs and Estimate Sales Velocity
• What ROI Minimum Should You Set for OA Deals?
• Q4 Inventory Planning for Amazon FBA Sellers
• The FBA Cash Flow Trap and How to Avoid It