Retail Arbitrage Profit Math: How to Calculate ROI Before You Buy

Most bad arbitrage buys aren’t bad finds. They’re good finds where the buyer counted two numbers instead of seven. Here’s the whole stack, in the order it comes out of your sale price.

Fee schedules change, usually annually and usually upward. Treat every figure here as the shape of the calculation rather than a current quote, and verify against the marketplace’s live fee schedule before you commit real money.

The seven numbers

1. Sale price — what the item realistically sells for, not the highest historical price
2. Referral fee — the marketplace’s cut, typically 8–15% on Amazon, ~13% on eBay
3. Fulfillment fee — FBA pick/pack/ship by size and weight tier, or your own postage
4. Inbound shipping — getting the item to the fulfillment center
5. Prep cost — poly bags, labels, bubble wrap, or a prep service’s per-unit fee
6. Storage — monthly, and painful in Q4 when rates roughly triple
7. Returns and shrink — a percentage of units that come back unsellable

Worked example: a real markdown

The item below is a real clearance price from our scan feed, pulled when this page loaded. The fee stack is the one above. Press the button for a different item — most markdowns don’t clear the bar, and seeing that is the point.

Sale price (assumed)          $129.00
Referral fee (15%)            -$19.35
FBA fulfillment (large)       -$11.50
Inbound shipping               -$4.00
Prep                           -$0.75
                          -----------
Net proceeds                   $93.40
Cost of goods                 -$49.00
                          -----------
Profit                         $44.40

Worked example: the trap

A cleaning spray, regular $12.98, marked to $1.30 — a 90 percent discount, which looks spectacular. It sells at $9.99.

Sale price                      $9.99
Referral fee (15%)             -$1.50
FBA fulfillment (small)        -$3.60
Inbound shipping               -$0.60
Prep                           -$0.35
                              --------
Net proceeds                    $3.94
Cost of goods                  -$1.30
                              --------
Profit                          $2.64

ROI is 203 percent, which sounds incredible. But the profit is $2.64 a unit. You’d need to move 40 units to clear $100, and that’s before the item turns out to be hazmat-restricted, or the price drops a dollar when two other sellers show up, or you eat a return. A deeper discount on a cheap item is usually a worse deal than a shallower discount on an expensive one.

This is the single most useful instinct to build: percentage off is a headline, dollars of profit per unit is the business.

The thresholds worth using

These are conventions, not laws, but they hold up:

• ROI ≥ 50% as a floor. Below that, one price move erases the trade.
• ROI ≥ 100% for anything you’re buying in quantity.
• Profit ≥ $3/unit minimum, and realistically ≥ $5 once you value your own handling time.
• Margin ≥ 25% as a sanity check against low-ROI, high-dollar items.

Both ROI and per-unit dollars have to clear. An item that passes one and fails the other is a skip.

The costs people leave out

Sales tax on the purchase. If you don’t have a resale certificate, you’re paying 6 to 10 percent on top of your cost of goods. On a $49 buy that’s about $4 straight off the profit — and it comes out of the thin end, where deals flip from marginal to negative.

Storage, especially Q4. Amazon’s monthly storage rate rises sharply in October through December. An item you bought in July and haven’t sold by November is paying rent at the worst rate of the year. See the Q4 guide.

Long-term storage surcharges. Inventory sitting past roughly a year gets hit hard. This is how slow-moving stock quietly turns from a small loss into a larger one.

Returns. Budget 3 to 5 percent of units in most categories, higher in apparel and electronics. A returned item is often not resellable as new.

Price erosion. The price you saw when you bought is not the price you’ll get in six weeks, particularly if other arbitrage sellers found the same clearance. Check the price history, not the current price — see the sales rank guide.

The velocity adjustment

Two items, both $40 profit. One sells in 10 days, one sells in 120 days. They are not the same deal.

A rough way to compare: annualized return = ROI × (365 / days to sell).

• Item A: 80% ROI, sells in 10 days → 2,920% annualized
• Item B: 80% ROI, sells in 120 days → 243% annualized

Item A can be bought twelve times over with the same dollar. This is why sales rank matters as much as the spread, and why experienced sellers will take a 60 percent ROI on something that moves over a 150 percent ROI on something that sits.

A quick mental version

For a fast in-aisle decision on Amazon FBA, this approximation gets you close:

Net ≈ (Sale price × 0.75) − $5

The 0.75 absorbs the referral fee plus a typical fulfillment fee on a standard-size item; the $5 absorbs inbound, prep, and a returns allowance. Then check that net minus your cost clears both your ROI floor and your dollar floor. It’s wrong at the extremes — very heavy items and very cheap items — but it stops you from buying obvious losers while standing in an aisle.

Frequently Asked Questions

What is a good ROI for retail arbitrage?

A 50 percent return on cost is a reasonable floor, and 100 percent or better is the target for anything bought in quantity. ROI alone isn’t enough — pair it with a minimum profit of $3 to $5 per unit so you don’t fill your shelves with high-percentage, low-dollar items.

How do I calculate Amazon FBA fees?

Add the referral fee, typically 8 to 15 percent of the sale price depending on category, to the fulfillment fee, which is set by the item’s size and weight tier. Then add your inbound shipping, prep materials, and an allowance for storage and returns. Amazon’s Revenue Calculator will price the first two for a specific ASIN.

Why is a 90% discount sometimes a bad deal?

Because fees are largely fixed per unit, not proportional to your cost. A $1.30 item selling at $9.99 loses about $6 to fees regardless of how cheaply you bought it, leaving a couple of dollars of profit per unit. The percentage looks enormous while the dollars stay trivial.

Should I include my time in arbitrage profit calculations?

Yes, at least informally. If a four-hour sourcing trip plus two hours of prep produces $120 of profit, you’ve earned $20 an hour before tax. That may be fine, but it should be a decision rather than a surprise.

How fast should an arbitrage item sell?

Faster is better because it frees capital to buy again. An item that returns 80 percent in ten days beats an item that returns 80 percent in four months by more than ten times on an annualized basis. Use sales rank to estimate velocity before buying.