Along the lines of the auction houses... I think it would be good to have in-house trading, selling and here "take this crap sword its better than what you have". You can accomplish this on the message boards and/or after a hard day defending the castle over a few pints. Peeps can make requests for items they are searching for, would like to sell/donate or what they would like to trade for. Even items they are not using for others to borrow or rent.
If no one wants the item or its rare and you want to sell for a boat load thats kewl, but be nice for guildies to have a shot at getting what they want or need instead of paying another guild
for it.