They're considered self-employment if you register through the government as self-employed. If not, they would be counted as "Other earnings" on tax forms at the end of the year.
The only time you would have to report earnings from them is if you make more than $600/year from art sales, which is $50 a month if you're consistant. I think that limit is raised slightly if you're registered as self-employed, I'll look into that shortly just in case.
Now making a few dollars over that isn't a big deal, but if someone was making a substantial profit and did not claim it on their taxes, worst case scenario is that they're audited. If you have your funds transferred to your bank account, they can view that ask you where the income is coming from. You can tell them the truth and maybe get a small fine for not providing accurate income information, or you can lie and get a hefty fine and maybe have to go to court for tax evasion.
I honestly don't think too many people have to worry about this considering the average most people charge for their art isn't a whole lot.