Well, most transactions are thru Paypal and they do have a dispute system if there are problems, though its not perfect. Mostly, since this is a community, you can evaluate a seller based on his/her reputation (the points and the concept) to determine their trustworthiness. There are quite a few good guys here... but, are you looking for user-built lasers, or commecial-built items sold by users?
For the process, I think most pay via Paypal (some Google Checkout) then the seller ships the item. It is possible to have another member act as "Escrow" where the buyer sends them the money and seller sends them the item and the "middle-man" verifies each end is fulfilled and completes the exchange.