This one is maybe a moonshot along with vendors accepting alternate currencies, but it would be pretty nice if vendors could have buy orders placed on them where the one running the shop can request X amount of a certain material at Y price. It would definitely help out people who don't have a vendor spot or just not interested in running a vendor to actually sell materials.
Another nice feature would be the ability to actually leave messages for the shop owners with the vendors - people do it in RP all the time, but of course the vendors never *actually* pass along the messages!