Both sedo and afternic do a lot of sales, so it's worth listing on both sites. Afternic charges a flat annual fee to list as many names as you want, that might keep some of the crappy names out from sellers who aren't serious enough to pay an annual fee. Looks like Sedo is going to start a buyer verification process, to weed out the looky loos. Afternic charges buyers a $1 fee to keep the non-serious bidders out.
Perhaps your buyers checked for the name they wanted on both sedo and afternic, and they prefer the afternic system? As a seller I prefer the afternic system, management area, the whole format compared to sedo. Usually in the dnjournal weekly sales report sedo has more top sales than afternic, so you really have to list on both sites IMO.