I know it's required to fill FinCEN 105 for $10,000+ flying internationally. However, I was wondering about my particular situation. I live in the US and plan to sell my art/merch in foreign countries at events/concerts when they eventually reopen post-COVID. I should fill the FinCEN form returning to the US but if I'm stopped and questioned about how I obtained the money, I would answer truthfully that I sold my art but is that something that could be deemed illegal or suspicious?
IANAL and it is best for you to confirm with a lawyer since this answer is not legal advice.
Yes and no. You are indeed allowed to bring a large amount of cash and you are obliged to disclose it. Doing so is therefore legal but the reason they ask to disclose large amounts of cash coming into the country is that this is often related to suspicious activity.
When you declare the amount, you will be thoroughly questioned and it is the custom officer who decides what proofs to ask and whether to escalate the disclosure. Bear in mind that, even if you pass through customs with minimal hassle, you are likely to be questioned every time you cross the border in the future even if you do not declare such amount on subsequent trips because this will be in your history. I know people for which this has been happening after making just one such declaration.
It is better to not fly with the actual cash and do a wire transfer. You will still be importing the same amount but there are fewer suspicions with traceable transfers than cash. An accountant should be consulted to know if you will have to pay taxes on your gains. This varies by jurisdiction but usually there is a threshold below which no tax is claimed and above that a lot of factors apply.
You must be prepared to justify your possession of the cash, i.e., demonstrate that the cash is the result of legal activities. Paperwork (ledgers, account books, bank statements, etc.) will be more persuasive than words. But it is impossible to predict whether whatever documents you carry will be enough to satisfy the authorities; there are too many variables.
Guessing wrong will have really bad consequences. Thus, it'd be best to not carry cash. If you guess wrong, and they think you're trying to pull the wool over their eyes, the currency could be seized by any of the border authorities you encounter.
You're much better advised to transfer funds in another manner.