Some things to be aware of on storage. The BMS in the battery draws a small amount of power, which, over several months, can become significant. What is much, much worse is that the power draw is not on the battery as a whole, but only on one parallel group of cells, and there is no protection whatsoever from drawing that group of cells down to 0 volts. Those cells must be removed and replaced, which is not a simple operation at all, unless you have a spot welder, in which case it is only tedious and difficult.. Those cells are not likely to ever hold a charge again, if they do then not for long, and they are reasonably likely to catch on fire when you attempt to charge them. Any new cells would not be matched to the other cells in the battery, which may cause some minor to significant issues.
Also note that while 0V is major damage, pretty much anything under 2.5V is some damage, and it gets worse as the voltage gets lower, and also the longer they stay low, the worse it gets, even without the ongoing drain.
The absolute assumption to make on such a battery is that it is no longer viable after prolonged storage. Also that the seller would lie to you.
One test would be to charge over night, after testing voltage, test again after full charge, then wait 2-3 hours and test again. It may not ever reach full voltage, if off by exactly, or very close to, 4.2V, you have a dead cell group. It may drop significantly after a few hours wait, that is the damaged cells self-discharging. Also, do the charge at the sellers place, before purchase, as their is some danger involved. Personally, I would not explain the full details of the test to the seller, but make sure there is nothing flammable nearby, or overhead. A garage with an internally exposed wooden roof would be a strict no-go.