Do you know the previous owner???? big help if you do. If a stranger, be very careful and enter any transaction with eyes wide open. Pop-ups are like any RV, they can be made to look very nice and still have serious hidden problems.
Have the current owner crank up the top and complete a set up and take down. It should go smoothly without the owner having to tweak the process with extra steps. Watch the fit of the door, bed braces, a level top. Do your research and have a good feel for the price range for this unit. Don't expect to pay "retail" which is the price you would expect to pay on a dealer lot. Units that are in tip top shape and were pampered by the previous owners can command a premium price, but don't go over the sale by owner price range.