To consider a resale price, look up the model on the
NADA RV website and see approximately what it's worth. Remember that you are looking during prime sales season (the beginning of the camping season), so you are less likely to get a bargain price than you would be in Sept-Feb. Pop-ups are in the section called "Park Models and Camping Trailers."
When looking at a used unit, ask if there are any tears, leaks or water damage.
Ask whether all systems function and request a demonstration of the lift system, water (in & out), gas stove, electrical and fridge/heat/AC (if equipped).
Check that the entire tenting and all connections are intact (replacement starts at about $600+++).
Look underneath and step along all flooring for signs of weakening due to water damage. Check inside cabinets and along ceiling/roof for signs of leakage.
Flip the mattresses and look for mildew or water damage to the wood end bunks.
Ask if there's an awning included and request a demonstration of set-up.
Avoid old-style canvas, in favor of the breathable tenting materials (early- to mid-1990's and newer).
Hope that gets you started. There have been other threads on this topic in the past, so do a search on pop-ups or PopUps to see what you find. And keep trying to log on to
PopUp Times. I get an error message for the forums right now too, but it ought to be fixed soon. It's an excellent resource. Enjoy!