I like the idea for a BTC based eBay on face value. As a BTC user and enthusiast, I foresee your problem being in variety which is what people will undoubtedly complain about especially when you are held in the light of eBay as a competitor. I'm a marketing guy and I've lurked a lot on BF but until a prospective client said I didn't have enough posts on BF to help his project, I didn't realize how valuable my voice is. Sort of brag side rant, but just explaining my low post number and old account.

Any mechanism you can embed in the site that incentivizes adoption or listing items would help your site scale and add users fast. For example, instead of charging fees for listing an item, make it either a fee or an amount of marketing work. For example, many sites give out affiliate offers for all of their users. LocalBitcoins is a great example. Their problem is that the reward for sharing the link is too low. If you gave users one free item listed for each person that clicks their link (up to the first 10 maybe) and then say 10 free items listed for each 1 successful referral. Then there can be an option to pay to list an item. I would recommend .001 BTC or so (something so nominal that it's just a place holder for a slightly larger fee as you need to scale). By setting the cost of listing so low, people won't feel disinclined to list. Not having enough listed items and by extension not having enough users period will make it extremely hard for a budding marketplace site to thrive and grow.

For someone to shop a store that deals entirely in second-hand goods, that store has to have variety and bulk supply.

Another point and I'll take a breather: Having amazing on-site SEO is going to help you get indexed and thereby bring in more users.

TL:DR- Heavily incentivize sharing & listing.