Hack, et al,
Just a thought, you may be going about this backwards. What you may want to do is try to compute the cost per day/week/month of running a site similar to eBay. When you're over that shock, you can then start looking at pricing structures and the volume levels required to cover the expenses along with whatever profit percentage you feel is needed.
Bandwidth, redundant servers, networking equipment, software licensing, web designers, programmers, lawyers, tech support, fees, lawyers, mediators, personnel, lawyers, accountants , etc., etc., etc. expenses are hard to pay for with free listings. Oh, and I think I forgot to mention lawyers? And then there's the three BIGGEST expenses for almost every successful internet only business; ADVERTISING, ADVERTISING & ADVERTISING.
Can you get a hold of financial statements for eBay? You need to do some serious ratio analysis on their numbers to get a better grasp of what you're up against.
You'd have to get your site volume up to a certain level before you can start attracting advertisers and their $$$.
Maybe you can consider some kind of flat 'Insertion Fee' along with a tiered flat 'Final Value Fee' structure that is reasonable and easier to compute. One thing that is annoying is having to pay the full percentage structure fees for a high dollar buy-it-now item. Yes, I realize that you are taking advantage of the eBay marketplace but eBay isn't really doing anything to earn the extra 1.5% via it's auction service enabling the price to be bid up. If I list an item for $50,000 vs. $100,000 via buy-it-now why should eBay earn an additional 1.5% of the $50,000 difference ($750)? They didn't really do anything more for the $100,000 sale than the did for the $50,000 sale, did they?
Another thing that really frosts me at eBay is their pricing structure is set up to facilitate the exorbitant shipping/handling cost scam/workaround. I would rather pay "fair market value" for an item and reasonable S/H costs than $0.01 for the item and $69.99 for S/H. It may come out to the same amount but psychologically I like the first scenario better. What really sets me off are items that are sold for FMV then still have totally outrageous S/H amounts attached.
Just some of my 2 cents. I'll stop before you get the whole $1's worth,
HayTay
Don't be the one that stands in the way of your success!