I'm not sure about having a point system, but The Tarantula Forum has a vender review section and newcomers are directed to that if they ask who to buy from in their area. Pretty useless for me in Australia but useful for most.
How they work it is when someone buys through the market section they either create or search for a thread with the sellers name and add their review. So even a lone sale by a member can get a review in the section.
Then when someone else comes along later and sees something advertised in the market section they search for the sellers name and see if there are any reviews for that seller and it will be there even if they've only sold one thing in the past.
The problem with a point system would be that people running a business, or people constantly buying and selling will get more points than the average hobbyist but that's not a true reflection of honesty and service, just more sales.