After playing the game for several years, you might find your storage overflooded with unused (raw) materials : planks, brew, stones, cannons, titanium plates, water... It would be great if these things could be recycled in -for example- gold coins or guild coins. The system would work in the same way as you can do for the conversion of brew into coins or stones and planks into tools etc.
Some players have such an overstock, they can never sell it in the trade office.
Price rating would have to be lower than the current market prices : 1000 fish 1 coin, 1000 cannons 10 coins...
Or we could leave it as it is, with a massive amount of unused materials in each player's warehouse.