first, DONT go for a free/cheap system. You're wasting your time, and eventually, you'll crash your system, and have to REWRITE your product databsae (wasting TONS of time)
second, DONT go for a linear system. Go for a graphical oriented system, NOT intuit quickbook, microsoft pos or myob pos.
so your choices are Radiant Systems Aloha, or ChikPOS. The price of ChikPOS means the Aloha system simply SUCKS (it's double the price, and half the features!)
even under YOUR limited circumstances i would STLIL go ChikPOS on a Windows emulator (you can get one for free) for Linux, because it simply is SO nice to use.