> Except the Credit Card Firm, my bank, and artists bank. All of which are required, by law, to keep records.

But they're not required to tell anyone whether a transaction happened.

> So if the artists try to go back on the agreement, he can be sued.

Sued about what? The supporters want the artist to keep the money, there are no legal conflicts here other than that the artist may lie that he/she is supported by particular people. Unlike in bank case, you and me and anyone else may see for ourselves who is supported by whom.