Hi Squamish and welcome to the Forum...
We all were first buyers once and hoped we made the right decision...
If I can suggest, first ensure the trailer fits your needs, wants, and your Tow Vehicle's tow limits...
Try to get the Trailer's "Build Book" with the trailer; this a compilation of manuals for all appliances, radios, TV's, Converter, heater, AC, TV Antenna, including running gear, etc. It should also include all the trailer specs and manufacturers included options.
In addition to what others have posted, here is a great site to look at:
Changing Gears has several links to look at:
RV Checklists
and once there, check out the link:
RV Inspection Checklist...
Checklist for inspecting new and used recreational vehicles, also known as the pre-delivery inspection (PDI).
I also included the Changing Gears linked document. The list from Changing Gears is quite comprehensive and there may be check-points that are not applicable to the trailer you are looking at.
Remember as you go through the checklist and your prospective trailer ... you are buying used and will more than likely find things are damaged and things that you find in need of repair... these should all affect what you pay for the trailer.
Necessary repairs can add up to more than the trailers worth, be very sure before you buy.
DON'T FALL IN LOVE WITH THE TRAILER BEFORE YOU BUY....
Good luck and if you are not real familiar with trailers, it will be in your best interest to take someone with that is and let him or her go over the checklist with you ....