Square's credit card reader technology is patented. Here's a claim from patent US7810729 for illustration:
1. A card reader device for reading a card having data stored on a magnetic stripe incorporated into the card the device comprising:
- a read head for passing a magnetic stripe of a card by to read data stored on a magnetic stripe and for producing a signal indicative of data stored on a magnetic stripe;
- a signal setting device for setting an amplitude of the signal indicative of data stored on a magnetic stripe; and
- an output jack adapted to be inserted into a microphone input associated with a cell phone for providing the signal indicative of data stored on a magnetic stripe to a cell phone.
In spite of that more and more companies have been using Square's audio jack technology (Payfirma, Payd, Payleven).
Would it be legal to purchase Square credit card readers off of eBay or AT&T and develop my own mobile applications using its technology? This is assuming I sell these applications as a SaaS or off the app store.