I once bought a used mattress /box spring set from a friend whose roommate had moved out and abandoned it. At that point, I never even thought about bed bugs, and I had no problems -- but it could happen, and IF you get them, they're a major problem.
I like the suggestion others have made about disinfecting the box springs, sealing it in a plastic case . . . and using it. Definitely slit it open and have a look inside -- look up pictures on the internet first so you'll know what you're looking for. Bedbugs aren't microscopic or anything. Do you have a place you could leave it outside for a week or so before bringing it in? Bugs won't stay where they have no food.
Finally, it is illegal to sell a used mattress here. Goodwill won't accept used mattresses either. However, there's a catch: It's perfectly legal to sell, say, a pillowcase . . . and let the person take the mattress away for free with that purchase.