Hi Bachus:
I admire your honesty and strength of character. Your question has many variables, making it difficult to offer an applicable reply. Would I buy a knowingly damaged Keyboard? It depends on why I wanted it. (Use it for parts, have the skills to repair it myself... or have access to someone who does, believe the damage is so minor it would not affect it's playability, and the list goes on.)
Good luck, not matter the reason. Personally, I would always investigate (if possible, to understand the damage) then negotiate to make the potential risk more palatable... unless the "deal" is too good to refuse.
Dont worry
I will only buy if the damage is just the outside dent
I will test the system toroughly
And have a tool with me to make sure that the bottom of the keyboard is not bent
I called yamaha and the small dent is no reason to void warranty in Holland
When you sit behind the keys the little dent is almost not noticable
If there is any doubt at all, i will not buy
And if there is no doubt, i will have made a great deal