Upvote:2
Yes it's completely safe according to your definition of safe. I have been to Rome many times and spent lots of time people watching there. Locals and foreigners alike have no issues with their purchases although, as some have mentioned, the products are typically low quality and overpriced.
in my experience the same is true of similar vendors in any city popular with tourists anywhere in the world. I have never heard of the things you are afraid of happening.