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

Visa and MC are not card issuers. They are card networks.

A card issuer is a person that issues a credit card, so major banks, and Amex who happens to be both a card issuer and card network.

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.