Sedo's double-commission structure is a disgrace, but not new. You wouldn't pay ebay a double commission just because your auction was won by someone referred through an affiliate link. Sedo should pay affiliates out of their own commission, not screw domain sellers on fixed price listings, where we should be entitled to know, before the sale completes, exactly how much we will receive net.
To avoid being stung. only listing domains on Sedo as 'make offer'. Then, when you get a bite, you can see if the buyer came from an affiliate (20%) & either up the price to compensate (& risk losing the sale) or try to subtly direct them elsewhere. You will also avoid rewarding any affiliates who are cookie stuffing.
Arguing the toss after a sale has been made is unlikely to be fruitful, as at some point your friend will have agreed to one of those "by continuing to use this website you agree to being f_cked by our new price list" type messages.
Let us know how Dan compares to Sedo, once you are established there.