I prefer a system with references, where I can make sure what the trades were like. It is not the same thing if someone has 5 positive (trade counts) ratings, where the total value of trading is not more than $50 as well as one trade worth $500.
As far as I know, only the admin and maybe some of the moderators can give a + or - trade count. That's ok as long as everything is simple and only happens a few times a month. Trades take place on Bitcointalk daily and it would be necessary to have moderators just for that, probably as a full-time job.