So how's the frame fit? To me, if the bike isn't the right size, that's a deal breaker.
If, and only if, the size is right, my rule of thumb for used bikes is 50% of retail list. That would make your bike 275. I'd offer the owner 250 and see what he says. You can always raise your offer if you choose. Another negotiating point might be "sweeteners." If the owner stands firm at 300, try to get him to throw in something extra - doesn't even have to be bike related.