1. Business model - in the future the $ comes from dealers and additional functionality (stores/featured auctions) not present on the site.
2. Traditionally the collectibles markets are tough because while you can supposedly sell something for $xyz, you'll need to actually find a buyer willing to pay that. Many items are worth a lot of cash but end up selling for next to nothing because collectors are very specific. What we're doing is basically a reverse-marketplace on the back end. Users say what they want and then dealers get a feed of the most "wantlisted" items and compete for the bounty of filling those wants. All marketplaces have to solve the chicken and the egg problem (as you went on to note re: buyers and sellers both needed) -- this is our attempt at overcoming it for both sides. There is incentive for sellers, as they'll get a near instant sale. There is incentive for buyers to stick around as they just add something to their wantlist and get a near instant offer. At least that's how it works in theory :)
3. You can use the transaction system without the site right now, yes. It's just not well documented. I suck at the documentation :)
4. Agree on all. The help and support section is brilliant, will be working on that right now. Facebook login is a step in the "user start" page that lists a few tasks for them to complete in order to gain trust in the marketplace. That's a big push for us, trust building.