> no one actually interacts with people selling you things like this

There's a whole successful social network (Goodreads) built on people's desire to talk about books, frequently in the form of effusive praise. I think being able to trace the product to a single individual (e.g. a book's author) helps make it particularly appealing to leave that sort of feedback: we know it feels great to hear that someone values a thing you've created, and we like the idea of giving that pleasure to the creators of things we like.