PCI[0] compliance might be a problem if you want to accept payment cards. Also, getting banks to recognize you as a payment processor is PITA (disclaimer: I work in POS development).
If by that you mean to simply add display on top of a cash register and pinpad, then yes, RPi will do just fine. However, if you want to manage payments as well, you must conform to PCI standard and get certified which is lengthy and costly process.
0. https://www.pcisecuritystandards.org/