I don't disagree that this could have many possible advantages, but I still don't think it will work.
Look at the way that payments are handled today:
1) Buyer pays through paypal.
2) Paypal transfers funds directly to the seller's paypal account minus fees (Paypal is 3% + 30c)(DB is 6.9%).
Your suggestion is to turn it into this:
1) Buyer pays through Paypal.
2) Paypal transfers funds directly to Deckbox minus fees (3% + 30c), and seller gets DB bucks of equal value.
3) Seller is free to choose when to cash in DB bucks for real money minus DB fees (6.9%).
Results:
1) Paypal gets it's fees no matter what.
2) Deckbox makes less money since some of the fees they would collect are tied up in DB bucks.
3) Seller gains the option to trade his/her DB bucks, but after cashing in would net the same amount of real money as in the current method.
Is that really what you want to happen?
Oh, and don't forget that the DB devs would actually have to develop it. That takes time and money too. Plus, DB would then be handling everyone's real money instead of just collecting fees which, I imagine, could be problematic.