Used cages are a good way to get cages for cheap, I would just make sure it's not rusted or broken. Also wash it down with bleach yourself, not just normal vinegar and water mix you would normally use for cage cleaning. Even if they said they already cleaned it, I'd still do it myself as well just to be sure. If it has any wood shelving in it I would just toss them, same with any other cage items, if it comes with any, that can't be disinfected.
Personally I also like to find out what happened the to previous resident and what it was (for used cages in general not FN specific), and if it died, was it sick. That way I know how deeply I really need to get cleaning the cage. Does it just need a wipe/scrub down with bleach or should I disassemble it and really get into all the nooks and crannies.