Hello @sussmeier congratulations on selling your artwork! This could be a hobby or a business, depending on how regularly you do this work and what your intentions are with it. Regardless of whether it's a hobby or a business, you'll want to keep your own records of all your income, whether it's through a payment processor like Venmo or Square, by cash, or by check. If you do over $600 in sales through Venmo or Square, you'll receive a 1099-K that reports the income to you. You can use the 1099-K to help make sure you're reporting all your income on your tax return, but you can report income from cash and checks that you tracked separately as well. You don't need to have a tax form to report it as income. If you're self-employed, you can also deduct your expenses for things like materials and supplies. 

Here are some other resources from us that you may enjoy reading!

Reporting Self-Employment Business Income and Deductions

4 Tax Tips for Money-Making Hobbies