This part can easily changed to take the money from the actual buyers, and returning it to them upon being outbid/winning.
Example:
Buyer A Bids 500k for a Banana = -500k from his own inventory
Buyer B Bids 510k for Banana = -510k from his own inventory
Buyer A's 500k will return to him + Notification saying "You have been outbid" etc
Great suggestions btw, but i'm pretty sure we won't see this launch without the "Mail System"