That is an older system and is obsolete in many ways. However, the Q6600 is fairly capable, even by today's standards.
I would offer $200 and not go beyond $250. Keep in mind, all risk is on the buyer. Just a suggestion.
Definitely, especially if you're buying off Craigslist. I would check and make sure the specs match exactly what is listed before buying. The Q6600 is still a perfectly capable quad core, but it is quite a bit old now, but for $350 you're not going to do much better.