what is bad about gold farming?
Couple of points, specific to WoW:
First of all, for a long time, gold farmers were generally blamed for making the game less fun for "typical" users. They took up player spots on servers, which caused a lot of servers to have queues. This forced a lot of people to have to wait to get into the game during primetime hours (6-11pm). A lot of gold farmers tended to bother other players to give them buffs/conjured food/open locked boxes, usually pretty rudely. And, players that wanted to legitimately farm for their own gold had a harder time, because gold farmers would typically plant themselves in common, high-profit spots and camp spawns.
Secondly, visiting and buying from a lot of gold-selling sites puts you at risk to have your account stolen or hacked, either via key loggers or poor security on the buyer's part. Your account is stripped and liquidated, and that gold sold to others, creating a cycle that gives a very bad reputation to both gold farmers and gold sellers alike.
Third, for a while, gold seller sites would spam people in the game advertising their services, both through tells and through the in-game mail.
Now, recently, these aren't the problems that they used to be. I haven't seen a queue on my server in months, ever since the expansion it's been easier than ever to make money (less incentive for anyone buy gold), and with the most recent patch, Blizzard instituted controls to limit the amount of spam. That doesn't take away from the fact that gold farmers have accrued very bad reputations, and people who buy from gold sellers are generally seen as contributing to shady business practices.
Real programs don't eat cache.