First of all: what problem are you trying to solve? You mention bar codes and better inventory control, which is a larger scope that just point of sale. Are you wishing to receive into inventory? Perform physical inventory? Audit? Generate purchase orders?
You mention Linux compatibility with barcode and other retail equipment: generally speaking Linux is 'compatible' with a great variety of equipment. I really don't think that will be an issue.
You mention datamining. I don't know the scale of your retail operations, so I don't really know what you mean by that. I assume that you want to know what particular customers are buying so that you can a) improve your ability to accurately order inventory, b) suggest items of interest to customers based on similar purchase patterns, and c) realize better margins by more intelligent up-selling. All of these boil down to good domain knowledge of your retail sector, good awareness of the market, and developing a good understanding of your customers. That is, ask yourself how much datamining is going to help before you go down that road. [ Do you have any kind of customer loyalty program (frequent purchaser discounts, points, etc.)
You mention output in a useful format. The format isn't that important. Understanding what needs to be reported using useful metrics is important, and the harder question. Transforming data into CSV is not that hard.
To that I'll add that I agree with some other posters as to whether you should produce your own retail systems. There is plenty of risk in such an endeavor, not the least of which is your implicit agreement to keep such a system running... forever. Of course, that's a nice inventive to do it well, too.
Since a few others have mentioned their credentials, I'll add that I produced and managed the development of a significant number of *nix retail applications for a Fortune 500 retailer.
Jim Greer