Why buy and sell stamps in the first place? Why not just meter the mail? The actual metering could be done by a debit card transaction, or even a vending machine, and the printing could be done by a standard computer printer
The USPS has an on-line setup where, after using a credit card, you can print out labels for shipping packages. Some have free Delivery Confirmation; you can insure at an extra cost. I occasionally sell on eBay and it is invaluable. I white glue the label on the box and just drop it off at the P.O. Out here in the sticks, the USPS is invaluable.