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?

[nonopr]

It is common sense that a (used) last year board if worth about half the retail price now, if you buy it from a friend
If you buy a (new) last year board from a store it should be now selling in the stores for 20% to 35% off the retail price.

[Farlo]

Hi Nakaniko, French dealers have a kind of official tariff for used boards/sails (Argus). A drop of 45% the first year is not surprising and then maybe 15% of the remaining value every year. A brand new board that have been used 2/3 times already looses 15%. However this tariff shows only five years back. Most boards between 5 and 10 years are traded within 200/500€ depending on pedigree and condition. Boards older than 10 years are in the sub 150€ range, notably due to the sensible evolution towards shorter/wider shapes in the early 2000's. There are a few exceptions like longboards or extreme wave boards which retain more value for specialists.

[windsurftr]

dont buy older three years equipments