[Ahmes]

Unless you already have the partnerships with the credit card processors and the capital to protect yourself from fraud (90% of the work) - I would release the design as open source and make it compatible with Square.

This will at least let hundreds of people build their own readers, and potentially educate most of them in magnetic stripe encoding.

Alternatively/Additionally the Square reader is miles away from ergonomic and by having another design out there you can make a small profit by selling more reliable readers.