I don't participate in the approving process, so I can't speak on that. I would say most sellers are long-time members and has been in the business for years. Many of us, like myself, have had previous experiences with them so the word of mouth goes a long way. A little research should tell you some history of the sellers.
Administrators usually gets contacted by those who has had bad experiences and they try their best to fix any issues. Moderator(s) are also assigned to the Buy and Sell forum to keep an eye on all sales threads. There are only so many sellers on this forum that if their name is marked multiple times by multiple members for being a terrible seller, the staffs would know.