IBM and NCR are two of the primary vendors in large retail chain POS systems.
If you're not already intimately familiar with POS systems in large retailers you have a very steep wall ahead of you. Not to discourage you, but this is not an area that you're going to come in with a better mousetrap and disrupt the marketplace.
Aside from the hardware and specialized OS that runs the registers, plus integration with accounting and inventory control, you also need to solve the service aspect. Best Buy for instance isn't just buying a POS kit from IBM, they're buying a 2-hour response time to service issues that cause any register to be down. They're buying the fact that IBM has parts warehouses across the US, so that any failed component can be replaced without delay.
It's been many years since I was last involved heavily with POS systems (IBM), but the software is almost the LAST thing they care about.