I can see one way around putting yourself in the middle of every transaction, but it's still pretty labor intensive and probably too distasteful for both buyers and sellers, unless they're really committed to the process. Require that customers provide a telephone number when contacting a vendor, then require that vendors provide a list of invoice numbers matched with telephone numbers from the business generated by the ad. (Doing it with e-mail addresses would make box-stuffing too easy, but with phone numbers it would be harder for unscrupulous vendors to skew the system.)

You would reserve the option to use the phone numbers to verify the veracity of claims posted. Knowing that any post about a vendor by either party, positive or negative, might be verified with a phone call could help keep things honest, or at least eliminate the most flagrant abuse of the system. You would still have to build SOME infrastructure, but nothing as odious (or redundant) as eBay.

As for the reader who wants to hire programmers on the cheap for one-off jobs - it's already being done. The itmoonlighters.com site is at least one place where you can do just this type of thing (they also have sister sites for creative work and office work.) Not sure if you want to compete with them or not, but I'd imagine if you do, you'd need something at least as elaborate as what they've got. Again, they're in the middle of each transaction to regulate integrity.