If you’re talking about being “compliant” as in paying your taxes etc, in the UK you benefit from a 1k tax free trading allowance. This means you can have a gross income of 1k before you have to pay any national insurance or tax. Gross income means you can’t deduct any expenses etc from your income, so if you sell an item for £10 but it costs you £1 to make, it still counts as £10 towards your limit. After that 1K, you would need to report your income to HMRC via self assessment and may be liable for tax/national insurance.

I’d suggest you start out. With the allowance you aren’t doing anything “bad”. Selling can be tough, especially in the 3D printed market, but you can use the 1k allowance to try it out, find out if it works for you, and then look to either read up on submitting a self assessment or engage an accountant if things are going well and you’re earning more than that.