When I sell online now, it is usually on EBay. I think EBay still has the most traffic of all the auctions. They have the most categories to sell in. Buyers on EBay have stopped buying at Good Prices, but this may be because of the economy.
EBay seems to have control over the lead spot in online auctions, though. They offer to me to list for free. Now, if it sells, I get the fees to pay. My standing is still good on EBay. I have sales, and then I go for a while without a sale.
I have had my experience at online stores, dealing mostly in antiques and collectibles. They are just too costly. There is where the old saying comes into place: it takes money to make money.