Look outs in priority order.
1. Can your truck pull the trailer in terms of weight? Use the trailer's gross vehicle weight from the DOT plate on the trailer -- usually driver's side front. The trucks tow ratings can be found in the Trailer Life tables by clicking
HERE. Provide a safety factor by discounting the ratings 10%, 20% if you plan on towing in the mountain or Pacific west.
2. Deformation of frame and delaminations of siding. Look signs of leakage or water damage in the interior including soft spots in the floor.
3. Age of tires -- trailer tires last 5 years. By 7 years they are dangerous. Tread wear is irrelevant. One can almost assume that you will replace tires on a 5 year old trailer.
4. Operation of appliances. If a private sale have the seller hook the unit to 120VAC and cool down the fridge. Check temps. Try the microwave, boil a cup of water. Run all the plumbing including the water pump, flush the john. Check the convenience outlets with a 3-light circuit tester. Light the stove, the water heater, and turn on the furnace. Raise and lower awnings if any. Run the A/C. If buying from a dealer, get a written guarantee for all these items.
5. If the tanks are empty try the knife valves on the gray and black water tank. Knife valves always work hard but they should work. If the tanks are not empty, one should wonder why they are not.
6. Batteries last 5 years. Propane tanks last 12 years based on the DOT cert date stamped on the tank.
Assumptions:
1. Items number 1 and 2 above are deal breakers. Do not buy an overweight or structurally unsound trailer. An inoperative fridge is close to one, those rascals are expensive!
2. You will need a class IV or V receiver on your tow vehicle and a weight distributing hitch system with anti-sway control. If anyone says you do not, that is the first sign that they are a liar.
3. You will replace the tires and batteries soon, unless you are buying a unit 3 years old or less.
4. In spite of all of the above, you will spend about $1000 fixing and tarting up any used unit.
AND FINALLY, never trust anything an RV salesman tells you. All assurances should be in writing and on corporate letterhead or form.