(now, things are somewhat different if you have a heavy hitting, expensive legal team on hand. Or so I imagine.)
As others have said, a domain name is probably not worth that much. I mean, once you have it, and you associate that with your brand in the eyes of the public, it's worth a lot. But it's your brand that matters, and right now, nobody associates the domain with your brand, so it's a simple matter to pick another domain. I mean, google.com? sure, the name is worth ridiculous sums now. But in '95? maybe $100. if that.
I know this is just a stereotype, but if at all possible I would avoid doing business with a domain squatter. If I did need to do business with a domain squatter, I would assume that they would try to scam me, and act accordingly. I would never give a domain squatter the trust that comes with a percentage ownership in my company.
Wait until you're profitable before trying to buy a killer domain name. You can't pay your hosting, rent, or employees with a domain name.
If you're dead set on this domain name, renegotiate a cash price. Domain Capital provides domain name financing, and Escrow.com will reduce your risk of being ripped off. If the seller refuses Escrow, he's definitely trying to rip you off. The domain name secondary market is rotten with fraud.
I'd say your best bet is to contact a lawyer (if you can't afford one, perhaps you have a friend in law school, or your parents know a lawyer that owes them a quick favor). You want to make sure that you cover all the "edge cases", like:
- What happens to the domain if the startup goes bust? - Will control of the domain be transferred away from the current owner? If so will it be transferred to you personally, or to the company?
Your product will brand the domain name not the other way round.
If I recall correctly this is what Facebook did. They started out as thefacebook.com and then made the switch once they had money/success.