First question to ask... Why is it being sold as a "parts vehicle"? especially if it runs. I have seen a lot of vehicles on CL that the owners do not have the title to being sold as "Parts vehicles". if he lost or does not have the title, walk away from it.
at 191k, I am wondering if the transmission is going out, and that is why it is being sold as a parts vehicle. that can be costly if you don't swap it out yourself.
If it needs some tlc and is otherwise a good truck, 600 is a pretty good price. offer him 5 and see if he will take it. Hubs are pretty cheap, you can get them at the junkyard for 40 bucks or so. if it does not have manual hubs, consider "upgrading".
Overall, the body looks pretty good in the pics tho, and for me working on the mechanical is easier than fixing rust...
AJ