1) End users don't know about them
2) End users that do find them see lots of names with no or low bids and in many cases there are overt tm abuses which compounds an image of listing sites as being "thieves lairs"
3) They have terrible customer service, especially during transactions
If I were to run them, I would get the customer service part right from the getgo.
I also would filter for obvious tm abuses and encourage members to help by having them flag domains that are obvious abuses.
Finally, I would look at end user awareness, maybe by allowing people to pull them into private auctions within the site rather than just having it as a listing site. So rather than having to have a parked page, they could link via an icon to the auction, some kind of "trusted" logo. I would absolutely definately not have homebrew appraisals on a site geared to end users.