I have worked with Retail Pro, Simple Retailer, and now Catapult software. For a beginner to write a fully functional pos software that would compete would be very hard with no experience with other softwares or the programming of them. If you are doing this for yourself and going to grow the system as you learn, go for it. You would be getting into setting up scanners, printers, inventory control, pole displays, msr's, and barcode printing. The softwares I have worked with have teams of programmers working on them. Good luck to you. I have seen it done before. FYI microsoft now sells a retail software called Quick Sell. Might be a good base program to start with and manipulate.
Just trying to help.
Bo Spruell
bo.spruell@klinecr.com