Card issuers (VISA, MC), Amazon, Microsoft, major banks, Ebay, Paypal, Due, Payline, Paysquare. A leveraged buy-out would enable many more potential takers.

To add to the other list (all plausible):

Google, dozens of international companies (eg Softbank or SAP), Walmart, Target, Costco, IBM, Oracle, and numerous others.

Basically, several dozen companies are serious potential buyers. And that's before getting into the secondary list, which includes companies like Intel, AT&T, Verizon, Comcast, etc.

I think a majority of their processing traffic goes through Wells Fargo. They would be a possible acquirer.