Never bought one used, but a good first point is to consider your main use case. If you will mostly be doing sit down driving, flying, etc. sims, something like the HP Reverb G2 is low cost with good resolution and inside out tracking. If you will be focused on more room scale games, a valve index is a great solution without getting over the top expensive, but will cost nearly double what the Reverb G2 does. If you need something like the latter, but new is out of your budget range, I don't think there's a real issue shopping for used.