Having recently obtained a fair amount of copper coins, I went to the stripmall, just east of the Ring of Brodgar, and spent as much as I could, but there were not enough things I wanted available.
I am mainly looking for piros and bagels, so if you were selling those, it's time to restock. I would also be willing to buy chanterrelles for about a coin each.
Also, thanks to the moneychangers, it's very convenient.