I don't think I've ever met in a public place for Craigslist trades. I've always gone to the seller's house. I don't think we've ever sold anything, but when we put something up for free online we just leave it outside and tell them to come get it.

Key word is always. It's not just about being observed (with or without knowing when and by whom), but also having no control over it.

Craiglist-mediated exchange is a choice. School, prison or workplace is not - not in any practical sense.