Could be perfectly Okay... A 28Ah battery is massive and can take a decade to drain from BMS if the pack was stored full...
But if it was stored near 0%, then some of the cells could be toast.
Assuming the cells were NMC, never used, and stored at 50% and a in a garage at ambient temperature of 25°C, the cell should have a remaining usable capacity of above 95% and a DCIR no more than 110% that of brand new. But that's before considering BMS parasitic current draw.... Wich is typically in the uA (microamps) range...
If overall the battery still sits at a voltage anything higher than 50V, i would buy it for a best offer of 400$. 300$ would be more reasonable. Then I would manually balance each cell string as the BMS drain is normally uneven, ofter targeting the first 3 parralleled cell groups...
But if I assume the BMS has a 50uA drain and the guy did not lie about the 2 years duration, then
50 uA = 0.05 mA = 0.00005 A (of BMS draw)
So in 1 hour, thats 0.00005 Ah drained out of 28Ah
So in 1 day, thats 0.0012 Ah
So in 1 year, thats 0.436 Ah
So in Two years, thats about 0.9Ah ...
So in 2 years, such BMS would drain 0.9Ah out of 28Ah. Meaning in two years it would drain about 3.5% of the capacity of that huge battery.....
So unless it was stored empty from the start, I'm quite confident the battery cells are still good.... But look up the specs of the BMS in that pack to be sure about the current it draws... Also, make sure the pack is made of good, brand name 18650 cells... Finally check to see how many cycles (from story, i would assume zero cycles) the pack has.... Also was it stored at 25 or 40 degree celcius ????
Matador