Don't know your experience level, so I will assume nothing.
You should also have a PDI (pre-delivery inspection) list and expect it to take awhile (several hours) going through the trailer. This will help you inspect the trailer and get a little familiar with the various systems on board. Ask to video the experience so that you have reference material. The idea of an inspection in also a good one - make sure the inspector is certified. All the paper work should be with the trailer. We have a small box with every component brochure and manual. There are resources on this forum for PDI lists. Also use web sites like RVT, RVtrader and ppl motorhomes to help you with market resources (prices and features) on used trailers. I would also check the date on the tires and not accept anything older than a couple of years.