Many established members do it. But many established members also don't make it a secret that they have multiple accounts and have left themselves neutral feedback to confirm this. Selling your account is also allowed and not against forum rules.
And even if they do, they are by their characterization,
established, they create quality content. They don't write some BS non-sense just to increase their post count and show to the buyer that these are “worked” accounts. By filling the first page of a thread with useless garbage that won't help the OP, you're being disrespectful to him.
Enrolling multiple accounts in signature campaigns is against the rules. It's cheating, and those who do it take away valuable spots from other members who would gladly particiate in the campaigns as well.
Yes, it's against the rules, but I doubt if a campaign manager would ever consider this ethical side of campaigns. I believe that enrolling multiple accounts is against the rules, because you can fill the required posts easily. Imagine that if you had two accounts, both in signature campaigns, doesn't really matter if they're in the same one; you could simply quote yourself and create discussions and thus, abuse it.
For me personally, it is not moral for someone to use it in such a way at all - because one way or another someone is abusing their position to the detriment of someone else.
IMO there's a lot of merit abusing, it may just not seem clear. For example, a person that merits a campaign manager for... paying him should be considered merit abuse.