I think it would be nice if there was an option for direct trade, rather than just buy/sell. Direct trade simply means the shop exchanges one item for another one (for instance it could trade 3 iron ingots for 1 gold ingot), or maybe even allow for a combined trade (for example 1 iron ingots and 5 currency for a gold ingot). This would actually remove the need for the buy/sell distinction altogether, as both can be expressed as a trade between currency and items ^^