Good plan, though you just delay the problem of finding users to test. You will want to test your user experience at some point.

1. Clarify your benefit to the buyer to figure out who your ideal tester is (your zero negotiation and and unlimited deals reasons are a good start).

2. Work out where these people are and go promote to them. So you need people who: are comfortable shopping online, already research purchases online, are cost-concious, are actively considering a car purchase, live in <insert geographic restriction>. Posting to petrolhead forums asking for volunteers and buying adwords for "car comparison <location>" would be a good start.

Sounds like you have some dealerships engaged. They have access to car buyers so can they help you find testers? They may sees a risk of leaking prospects to the competition, but surely you can control who sees applications in your test environment?

3. Offer incentives, or you'll waste time battling the drop-out rate instead of fixing bugs. E.g. Fund your dealers offering a small extra discount for any sale completed through your service with feedback given.

Hope there is something new and useful in there.