And I reject the idea that making the larger vendors pay an annual fee curbs scammers in a substantial way. First off, the large scale scammers like Potts are relatively rare, its hard to maintain an on-going scam in a community like an internet forum. Second, for the ones who manage to do so, like Potts did for a while (even though I said all along the guy was a prick), the money they bring in is virtually all profit. Some minor fee they paid out initially is insignificant. And using your own logic, that fee would actually help the scammer earn some amount of trustworthiness simply for having paid it.
And the smaller scammers, the majority, will scam one, two, maybe 3 people, then move on to a new forum to start over. These people will be unimpeded by the annual fee completely.
A purchased vendor account is simply a money-making device for a site owner, nothing more.