Of course this is very subjective and there are a huge amount of variables - it depends, etc - understood. This is just for the sake of discussion, not locking you into something. If you were to buy a used lens, and you believed the lens was in working order, free of damage, and testing it contributed to that belief - what fraction of the len price brand new would you expect to begin with as you negotiated? In other words, if you were selling a lens you know is in perfect working order, you would expect to sell it for between XX% and YY% of the price for that lens new today. You would start out offering it at YY% and be willing to take as low as XX% for it.