I got lucky this time. They made a common mistake: In a 24-hour period, they all traded gold to the same alt, and while the alt belonged to a different account than our four farmers, that account shared the same credit card with Qrstuv. When we see that kind of behavior, it's banning time, and this case was no different.
Granted, maybe we're only catching the really dumb farmers. It's like that saying about crime: The best criminals are the ones you never hear about. There are almost certainly farmers who are so good at what they do I don't notice them. But if they're that stealthy, they're probably not disrupting anyone else's game, and they're probably not unbalancing the economy. And if they're not doing either of those things, I'm personally fine with their continued existence. I really don't believe there is anything intrinsically wrong with selling virtual items second-hand on eBay or wherever, despite the fact it cuts into a potential source of revenue for the game developer. As long as it's still a "potential" source, and the developer isn't making a concerted effort to actually get involved in the market (see Sony's Station Exchange for an example of a pretty good effort), I don't think the developer has the moral high ground to strike down any secondary markets.
Then there's the question of the game designers' intent. Having worked with designers, I know they hate farmers more than a lot of players do, because the farmers are finding and exploiting design weaknesses in products they've worked on for years. Some people would say, "Hey, tough luck. If your design is weak, it's your own fault." Others would take the stance that people who agree to play a game are entering in an implicit contract to not only play by the rules, but to play reasonably within the designer's intent. I tend to side with the "tough luck" crowd, but I would add that if you're going to exploit a weak design, you'd better do it in a way that doesn't undermine the enjoyment of other players.
In the end, it comes down to a cost-benefit analysis: Is banning a suspected farmer worth the $15 per month subscription fee you lose that the farmer is paying, and on the chance that it's not a farmer, is it worth the bad word of mouth to insinuate you don't trust the people who play your game? It's a delicate balance that involves the marketing and customer service departments of a company every bit as much as it involves the designers.
The attitude a developer takes in regard to farmers should be consistent and should be integrated tightly with the game's design. When we set out to design an MMOG, we should be as concerned with our policies and attitude toward farmers and other exploiters as we are with the art style of our game world, or the pacing of combat. If developers set down some guidelines about farming early on as a core part of the experience, the designers can take that into account during the development process. At that point, the developers will hopefully have a consistent and sensible set of policies about farming the community understands, and then maybe we can go about catching the right farmers for the right reasons.