Well definately listing fees, but as I said, only for the first 50 listing under the conditions stated.
Not sure on your costs for running the site, but if you can manage to offer free closing fees for that initial period even better.
You've got to see the bigger picture in that sellers might be prepared to list stuff and see it sell for less than they would like if it ain't gonna cost them owt.
If there fresh listing attracts bidders, and bid prices increase they should return with more chargeable listings later on.
If not, they have still given your site a bump with the extra listings and hopefully the increased bidding.
As I said, I don't know your associated costs, so don't know how feasible this is.
What I do know is it will be nigh on impossible for you to expect sellers to pay to list items that may not sell for lack of bidders, and even harder to attract more bidders without more stock at starting prices that could result in a potential bargain for the bidder.
That really is the chicken and egg scenario.
Also, it might be worth contacting promoters about offering some sort of free advertising for their events on your site if they incorperate your logo on their flyers......you need to get your site name out to all and sundry, and whilst Soul Source and similar sites is a great starting point, there are hundreds of flyers out there every weekend that could have your logo printed on 'em.
Again, not something that you need to do long term, but will all help to gain initial interest in the site.
Think you need that initial 6 month run up with such offers/freebies etc, and then it might hopefully just kickstart the whole idea to where you want it to be.