used sails and board price evaluation

[nakaniko]

I'm wondering if there is a system to calculate a the value of an used sail and board.
A friend, former windsurf shop assistant here in Italy, used a system that takes away from the retail price the 40% for the first year, and then the 10% every year more; but not sure if this 10% is about the starting retail price or about the price of previous year. The first takes the value to zero in not so many years, the second gives even more advantage to the seller.
Is there around a common system for a starting point evaluation, obviously then corrected by some factors?