Fixed fee highly favors big players. Not even sure why you want fixed fee. Either remove fee at all or charge higher for bigger players or charge based on sale rather than listing.

By the same I mean equal non-discriminatory pricing - not necessarily "fixed" rather than "by sale" or "by view" or what have you but that if it's "by view" then it's "x cents per view" with the same x everyone and if it's "3% of referred sale revenue" it's that for everyone.

The purpose being that because every item in any paid directory has paid the directory the same, the directory has no (monetary, at least) incentive to direct your attention towards sub-optimal listings. As an attempt at forcing the directory to sell itself as a useful directory of services, rather than as an object which sells its users attention to the highest bidder.