I think the old school DMG had suggestions on money changers and gem dealers where they charged 10% the total cost. So if you have 1000 gp and you purchase a 1000 gp gem with it, when you travel to town and want to convert that back into cash you only get 900. I like that because the gem cutter buys a rough gem, works it into a fine gem, sells it for a profit based upon the work he put into it, then makes another profit when he buys gems off future adventurers. Makes being a gem dealer lucrative.