I am cash basis and sell product through a third party. For my sales in December, I received payment in January. Do I list these sales and income on 2020 or 2021 taxes?

It depends.

If the third party is your agent, then he is deemed to have received the money on your behalf in December 2020, and those sales are December sales and should be reported on your 2020 tax return.

If the third party is an unrelated party who only pays you in January 2021, then those sales should be reported on your 2021 tax return.