Depends on the market kinda, state fairs and the like are pretty rife with these, most of the farmers markets I go to are hard enough to get into so its legitimate local products.
The ones I see a lot of are the rocks with wicks in them and salt lamps.
I'm not sure how the rock candles are a scam but I see the exact same ones at all sorts of markets/fairs and the like so I feel like something's up and it's not just people all making the same craft. I could be wrong though.
The salt lamps I'm pretty sure are a scam, but I kind of want one because they're pretty.