Wonder Computers had a *stack* of these at one point. They were quite difficult to get to work - I'd say that 95% of Amiga users wouldn't be able to get it to work - I couldn't. If memory serves, it took three of us togeather to get one working.
If the seller says that he can not confirm if the LCD works, then it is useless/worthless. I would say offer him $50 and if you can get it to work, send him an additional amount.