It depends. The problem could be a relatively simple adjustment and/or seal replacement, or it could be frame damage that has caused things to go out of alignment (less likely, but possible). It could also be a hydraulic or electrical problem with the system that powers the slide.
If you're really interested, look for an RV tech in your area that will inspect the slide, or have the seller take it to a dealer who services Open Road to have the slide inspected. Have them inspect other systems while you're at it. It will cost you a few bucks whether you buy the unit or not, but it would be money well spent to avoid major problems.