a hat or jar that a player can drop, most likely using a special drop verb so as not to cede ownership.
other players would interact with the hat/jar by placing currants* into it, but could not pick up the hat themselves.
currants* placed into the hat/jar would be immediately sent to the owner of the hat/jar.
the hat/jar would stay on the ground for no more than 10 minutes at a time. at the end of the timer, the hat/jar would vanish and reappear in its owner's inventory. existing "auction" rules would be used for dealing with full inventories and offline players.
*or, possibly, items.