After thinking it over I think I'm going to go for a small yearly subscription fee instead of the big flat fee up front. Unfortunately I am going to have to postpone the launch of the site as my aunt died from cancer earlier today :-( and I have to go to St. Louis for her funeral and won't have regular access to a computer or the internet. I was going to launch on 4/20 but now I think I'll move it back to next Monday.
Just to give you an idea I once researched the model of ebay. This is how I see it. Some may see the fees of ebay are high, but they actually like the high fees because it eliminates all the junk you see on other sites. You don't see the $0.01 for a piece of trash auctions. Whenever a legitimate competitor comes around they buy them out. As long as the fees are not 1/3 higher than shipping fees there is no way anyone would use another site due the quality, insurance, feedback, and paypal they have to back up the auctions.
1/4, 1/2, or even 1 BTC to do an auction, is not a bad price if you act as insurance and arbitrator. If I was going to sart I would do a 1/2 BTC fee, 7 day auction, and if the item didn't sell I would take the fee. But offer insurance and arbitration.