I like Durb's post. Most used trailers don't get used that much. Avoid the ones that may have been lived in at construction sites or full time. Notice the amount of wear on the floor and upholstery.
Have the owner demonstrate the major appliances. Refrigerators are one of the expensive things to replace. Nearly everything else can be fixed if the coach is clean and not beat up. Ask about the age of tires, repacking axles, etc. See how much maintenance has been done on the rig.
Buy the best quality you can find. Negoitiate with cash.