I've been running the local board game math trade for about 6 years now and like most people go through OLWLG which requires a BGG account. We allow item trades and cash trades (Cash/venmo trades need to be sent before the trade day otherwise its mass confusion). Recently I've been considering trying an alternate system which would not require a BGG account at all and our team has been debating the merits of requiring it.
For me, requiring the BGG account is helpful because it links a person to a "public" established profile so its less anonymous which would help prevent no-shows or people taking money and then not showing up. Also people that get cold feet or make a mistake and then decide they don't want to go ahead with it.
The flip side is it restricts people who don't have or don't want to have a BGG account. We have several people that participate in the post trade auction (run by a friend) that only do that one because it doesn't require a BGG account.