With their profile, you get to know the person's name, see their pic, get to know about their activity. They are people like you, who have been using the product. It feels more comfortable buying from such people, than from anonymous sellers, who could sell you defective items.
Each person is a part of a community, be it local community, or corporate or apartment. There are many other people who know them and who vet from them. The trust is not just based on a social profile, but also from people known to them.
In real life, we love to browse through items, window shop and if some things fancies our interest, we buy them. Similarly, browse through categories, window shop the items being sold, and read the comments people right on them.
If something fancies your interest, comment on the post, and chat with the seller. Finalize the deal on a chat, and buy it. In the same way, you would do a deal in a shopping mall.