Most "how to calculate eBay profit" guides stop at sale price minus fees minus what you paid. That is not profit. That is a number that will be wrong by the end of the week.
Here is the full subtraction, done on one real item, with the lines most sellers forget.
The item
A DVD box set, bought in a bulk lot, listed at $21.71 with free postage, promoted at 20%. It sells on a Tuesday.
Seller Hub shows you $21.71. Here is what actually happened to it.
Line 1: Sale price
$21.71. This is the number eBay puts in your face. It includes any postage the buyer paid, because eBay charges its fee on the whole amount, so if you offered free postage it is just the item price.
Everything from here down is subtraction.
Line 2: Final value fee
eBay's cut of the sale. On eBay.com.au for a business seller on a Pro plan it is around 13.4% including GST on most categories. On eBay.com it is around 13.6%. Some private sellers pay 0% under eBay's free-selling offer. The percentage depends on your site, your category, your plan and your seller type, which is why a fixed cell in a spreadsheet is always a little wrong.
On this item: minus $2.88.
Line 3: Per-order fee
A small fixed amount on every order, typically $0.30 to $0.40 depending on the site. It is tiny on a $200 sale and real on a $12 one. It is included in the $2.88 above on this item, but check yours, because it is easy to miss on cheap stock.
Line 4: Promoted listing fee
This is the one that kills more profit than any other. If the sale came through a promoted listing, eBay charges the ad rate on the sale amount, on top of the final value fee. At 20% on $21.71 that is minus $4.34.
Notice that the ad fee is bigger than the final value fee. That is normal at the ad rates people set to "just get it moving". A listing that sells only when promoted is not a $21.71 item. It is a $17.37 item with extra steps.
Line 5: Postage you paid
If you offered free postage, the label came out of your pocket. If the buyer paid postage, you still paid for the bag, the tape, and sometimes a second label when the first one was underweight.
On this item the buyer's price covered postage and the label cost was $0.00 because it shipped in a combined order. Put your real number here. For a $12 sale this line can be a third of the sale.
Line 6: Cost of goods
What you paid for the item. This is the line eBay cannot know and the line most sellers guess.
If you bought it on its own, it is the receipt. If it came in a bulk lot, it is that item's share of the lot. A flat split gives every item in a $100 box of 40 the same $2.50, which flatters the cheap ones and punishes the good ones. A weighted split gives pricier items more of the cost.
On this item, its share of the lot was minus $7.00.