IF you can go look at it and see it work your are way ahead of the game from not having to deal with ebay and what United parcel service ( ups ) would do to it when it gets shipped. THe odds of them getting a recorder to you that they dont break or destroy is a miracle.
Check it out the best you can, Record on all channels and play them all back. See if he has something recorded you can listen to.
If he has the manual that is a big plus and is needed on this recorder. If he doesnt have one Tascam will send you one for about 30.00 total, But its worth it.
Its a nice piece of gear to own, I wouldnt trade mine for the world.
As far as what to offer him I would start at 300.00 or less and see what happens. You can always go up in price but never down. If its in good shape I would go as high as 600.00
Good luck
Analog is the best!