If you're selling at ARTE, it's not illegal as the market gets its own licence which covers all vendors for the day and that includes food, though I believe the municipality does do spot checks to make sure that it's all hygenic and the the rules are being followed. There are specific guidelines on the ARTE website about what's required. For other sites and markets I really don't know, but if they're covered by a municipality permit, then there should be no problem. If selling from home directly to the public, then yes, a trade licence is needed and the site where the food is made would come under strict scrutiny by the municipality and a whole new can of worms could be opened. If it's an informal arrangement between friends, and no money is seen to change hands, then who is going to stop it?
