It depends on where you live. Many public places will require you to have a permit for sales. However, in a nice, family-friendly neighborhood as a kid, it would likely be fine. If you plan on selling street-crafts for a living, find out what the local regulations are and look into the costs of a stall, half shipping container or other form of suitable "pop-up shop" because you will be out there in all sorts of weather and you'll need to protect the crafts.
What’s the catch? None, really. Cash back apps act as affiliates for many online merchants, which means that whenever you make a purchase through one of the apps, they get a small commission — but then, they give you a portion of that commission as “cash back”. For example, if I buy a pair of Nike shoes through the Ebates app (or website) and spend $75, Ebates may get a $10 commission but then they’ll pass $7 back to me. It’s basically a way to get sale prices on stuff that isn’t on sale!