I think that selling prices from private parties drop quite a bit more if rig is 10 years old vs 2 years old. Online asking prices for 10 year olds seem optimistic, online dealers will probably dicker. In any event, you need to make sure that everything works and get a written estimate for any repairs/replacements needed to make rig fully functional BEFORE you sign. If repairs/replacements are to be done by dealers, have all promises written into the sales contract with a time limit including legal phrase " Time is of the essence."