I have thought of implementing this before, mainly for a lottery style item, where users purchase lottery tickets, and the sales go into a lottery user account. then the lottery is drawn, and then winner receives the winnings from the account.
I would imagine that to implement this, you would have to go into the shop-buy script, and find where it takes the users money, then add a quick db query to add the same amount to a particular account. Daniel15 or Basil would be able to confirm this.
*Edit* Damn, beaten to it. Told you daniel15 or basil would be the ones!!