Isn't it illegal without a license?
I've done it a few years ago, most bars have stopped selling it now. Even when I did it, you had to be very careful. I was always told it was the bar who would get a fine if I did get caught. Whether this is true sbd it is a licensing issue i don't know. Never did get caught though.
Easily profitable though. In September I could sell 50-100 balloons at 5 euros a time in about 3-4 hours. I know people who've done July and August and averaged about 200 balloons a night. Even if you make 1euro of each like most the girls do it's still a well paid job. The big winners are the people organising it though. Easily clearing 3 euros profit a balloon after paying the bar for doing none of the work!