Can credit card agreement or debt be legally transferred or sold to a buyer?

Yes, but depending on your location the actual laws involved change.
That's how debt collectors operate.

Credit card accounts are sold all the time. A credit card issuer has three choices on a defaulted account. They can try to collect using in house collections people, they can retain a collections agency to collect the debt for a fee or they can write off the debt and sell it to a 3rd party.