Maximum FVF on eBay is $250, but the standard rate is 9.9% of total price paid (unless in a special category like real estate, or motors).
PayPal is 30c + a percentage, based on the currency sent. (In other words, if the price is in AUD but the buyer funds a payment in USD, you'll receive AUD but be charged a slightly higher percentage). If the buyer sends AUD, then it will be 2.6% of the price, so $520 (+30c) on $20k, or $260 on $10k. (PayPal do have some fee caps, but pretty sure they don't apply to commercial transactions).
To work out the exact fees, it's easy enough to use a basic calculator with a % button.
For example, say you want to work out what will be charged on an $85 sale (including postage).
Type 85 x 9.9 then press the % to get the result (8.415 - probably rounded up to $8.42 when the fee is charged)
For PayPal, same sum, different percentage (85 x 2.6 press % = $2.21, + 30c, total $2.51, add that to eBay's fee of $8.42, total fees paid on $85 is $10.63).
