You buy an account, presumably, to increase your pay. Let's consider two scenarios, shall we?
1) The buyer has a Newbie account or no Bitcointalk account and aims to participate in signature campaigns. You can conclude that their post quality will not be great considering their experience. Moreover, their crap output is amplified due to less wait time between posts.
Newbie and Jr. Member accounts require merit to rank up. Buying an account does not. This means that they won't have to write quality content, since they're already at (presumably) a rank that can participate in campaigns. If you want proof, jump into any thread outside of Reputation and Scam Accusations.
2) The buyer already has a Bitcointalk account and simply wants to buy another account to increase income, regardless of its rank. This means an increase in required output of posts. I know you vehemently denied the fact that more posts = lower quality, but let's face it.
The majority of account traders are probably going to put in the same amount of time (at least for two accounts) and compensate by dropping the quality. You can often see the effects when you stumble into threads that have general titles that can be responded to within a minute.