It sounds like he is about $50 high and you're about $50 - $75 low if its in clean shape. If its rough, then I understand $350.
Remember, whaqt he buys for wholesale, he has to sell for retail to stay in business. If he sells too low, he he cant make a buck and then he's gone and you have no choice but the auctions or new.
Offer him $375 cash and see what he says. Ruger is still making them so he may have wiggle room depending on what he paid for it but dont be surprised if he says no. Have $400 cash in your wallet in case of that.