I think a lot has to do with how the previous owner maintained the trailer. Lack of maintenance could destroy a high end trailer just as easily, so look for signs of neglect. Maintenance records and receipts go a long way in making a good case for a trailer. Look for any signs of water damage inside and out, push on the exterior walls, check all appliances, etc. There's a lot to consider when buying a used trailer. Take your time and don't take the sellers word on anything. There are many good deals to be had, but with anything you need to do your homework and be cautious.
