That's because it isn't 20%. Following is GB's fee schedule not including any optional services.

Final Value Final Value Fee*
$0 $250 6.0% of the final value
Over $250 6.0% of the first $250 ($15.00), plus 3.5% of the remaining value

For comparison, my all-in costs for selling on EBay including PayPal fees is approx. 10% - not 20% as an earlier poster stated.