Did you talk to the seller? Did you test-charge it at the point of sale to confirm it can actually charge? If not, and there is no good return policy, I'm not sure there is that much you can do to get your money back, since technically you could have caused the damage by improper use after you bought it and it would be hard to prove otherwise.
If you rode it for 20 miles, that means there was some charge in the batteries when you bought it, so the batteries couldn't have been completely toast at that point. Not being able to go over 60mph indicates (most likely) the batteries were not fully charged to begin with, or worse, they had cells out of balance and the BMS was trying to protect them.
Question to more knowledgeable folks here - does the '13 FX use the round cells or the pouches, and is there per cell BMS monitoring (or per group of cells)? Granted, if the previous owner did not charge it recently and if there were a cell or two that were too low, my guess is the BMS might not have been able to protect them when you discharged it to "empty". Especially, if you turned it off, waited a bit, then turned it on again to roll a few hundred feet until it died again and so on, I suspect the BMS might have not done its job well and allowed battery damage.
Lastly, weren't the '13 subject to some battery recall? Or was it some '12 models?