Is it scratched?

If not, things like tires and grips can be replaced with exact new ones for a lot cheaper to put it back in new condition. If you fell and scratched it, well... I can see their point. My suggestion is to rent first, when possible. Spend a day or a weekend on it. If unsure, rent it again, till you are.

I've bought many bikes that were too big. Each one I've sold on Craigslist for a little less than I paid until I figured out what I needed in bike geometry.

I agree that stand over is not the best indicator of MTB sizing. Reach is the important one. Proper reach while the saddle is in proper relation to the cranks for power output.