I'd offer the guy half but no more than 3k for it. If it's not running and can start on it's own and the owner hasn't had it started since he's owned it for whatever reason, the guy knows how to replace batteries or at the least use a battery charger. Sounds like to me he is hiding something so if something is bad like a bad bearing or what have you, he can claim he had plausible deniability and you bought it as is. If he wants the 5k for it, I'd tell him to get it running. Otherwise there have been a lot of other 2.5 tons for sale in that price range that run and drive, even if it's an hour or two away, it's worth it for a solid truck vs gambling and we all know it's the house who more times than not that wins the gamble.