These questions are targeted to those buying used PS2.0 MTH locos, but can be tailored for other applications.
1) Can you provide DCS odometer/chronometer reading(s)?
If not, has loco had uses other than a home/residential layout? You probably don't want a dealer showroom or train show display item, unless it's a steal.
2) Original packing (styrofoam cradle, railing & truck protectors, foam and plastic wrap), item box and paperwork provided?
3) Condition of battery? This is very important; many are very difficult to replace.
4) Any mechanical or cosmetic issues?
5) Has loco ever had to be repaired?
If yes - reason for and outcome of repair.
6) Why are you selling this item? (a bit nosey, but can provide good info if seller answers)
7) Are you the original owner?
8) Last time loco was run, and all operating features were verified?
Ask specifically about smoke unit - lot of guys don't use smoke, so they really don't know for sure it works.
9) Smoke free environment? You can't imagine how bad the stink a loco from a smoker's home can have; and it takes weeks to rid the item of the smell; sometimes it never clears up completely
10) Please verify that loco will be packed securely, beginning with it's own interior packing. If there's a lot of slop in the interior styrofoam container, then the item gets slammed back and forth every time it gets dropped, kicked or thrown by your favorite package delivery service's employee.
Please feel free to expand upon this list.