You're missing the point *again*
Raising your prices in such a way is utterly counterproductive, especially with your rationalle.
Why not go the other way and reduce your fixed price to 7.5%?
Promote business not stifle & drive it away!
And 50E on 300E is 16.6* %. Not exactly the best of examples. The high minumum commission on small sales if one of the major reasons I'm not with you now. For small sales your proportionate commission is utterly unaccepatble. The straw that broke the camels back was a small sale which after your commission & would have been left with under £15 while you took £50! Ridiculous!
What about big sales if you are lucky enough to sell a £150000 domain you pay £22,500 just for putting it on sedo's site and the seller would have probably found the domain owner anyway. Bigger sales should be on a sliding scale unless sedo take an active part in finding the buyer, and the name isn't pointed at sedo. And how do you find a fixed price for a domain the whole object of putting it up for sale is to test the market. If you have a fixed price you might aswell put the price on a landing page in big text and pay no commision.