Tom -
There is no real checklist for things to ask a seller before buying. A few of our members started to put one together, but the info is so scattered that it is hard to pull together. The checklist that B&D linked to is Ed Felker's famous checklist, the granddaddy of all lists here on the forum. As I recall, it is a getting-ready-to-go-camping checklist, but it will contain a lot of good info for before-buying, as well.
There were a couple discussions about building the checklist you seek. Check out
http://trailmanorowners.com/forum/showthread.php?t=8893
and
http://trailmanorowners.com/forum/showthread.php?t=8878
Hope this helps.
Bill