Marketpress - Reverse, "Buy" Options, Inventory Login

I think I have my ideas for a POS system down using Marketpress, which brings me to an important feature (for me): The ability to purchase items and record these transactions and at the same time have them logged into inventory.

In essence, this would work in reverse: kick over to a "Buy" mode (or even better have a "Buy" toggle in-line with the item itself) where the quantity is added to the inventory at the end of the transaction. The inline would allow me to do in-transaction "trades", which would then subtract from the total value.
IE
Customer puts Widget A in the order at $20, and is selling me/trading in Widget B at $10; the order would subtract the $10 from Widget B's buy price, leaving $10 to be charged. Transaction closes out and Widget A is reduced by 1 while Widget B is increased by 1.

I buy and sell used video games, trading cards, etc. so this is important functionality for me. Allows me to quickly address my customers and track inventory real time without a lag.