We just bought our second used pop up, a 2007 Fleetwood Arcadia a few months ago. And we still own our older Coleman Niagara, which we love but will sell come spring. What we primarily looked for is this:
1. Floorplan - did we have enough beds? How much storage space did we have / need? We spent significant time looking at all brands and layouts to figure out what we needed, what we wanted, and what didn't really matter.
2. Check everything. Look at the roof and the floor for signs of water. Look around the seal between the roof and the box to make sure it seals properly so you don't get water intrusion while driving.
3. Check all the systems - make sure the heater works. Same for the water pump, water system, air conditioner, toilet, water heater refrigerator, stove, battery, electricity, etc.
4. Inspect the canvas for tears, holes, etc. New canvas is about $1,000 - you can order it but this will figure into your price if you find a popup with rotting canvas.
5. Check the tires / wheel bearings / tail lights. You need to make sure it is road worthy. Same for the braking system if it has one.
The biggest thing to look for when buying a popup is making sure that you can tow it safely with your vehicle. Pull out your owners' manual and check the numbers. Don't go over your vehicle's tow rating - it is for your safety and the safety of others on the road. I've seen too many accidents that make me very aware of abiding by tow ratings and tire pressures and using safety chains, sway control, etc. I know that one member on here had a bad accident a while back (maybe Minkydog??) while towing. Do your homework to make sure your family is safe.
Another good resource for all things popups is
www.popupexplorer.com. They have many forums with all kinds of posts on everything popups. The members seem to be very friendly and have a lot of good information. I have also seen checklists for things to look at when buying used (or even new). Personally, I have found used is much better - some really great deals on units that haven't seen much use. Sometimes people buy rvs and then don't use them more than once or twice a year. In many cases, they are in "like new" condition for 30% or more less than new.
Have fun and see you on the road.