Possibly someone wants to leave Bitcointalk and he want to sell account but very few cases comes. Its hard to rank up account and I do not think after a such time spending and working on account anyone will be agree to sell his account cheaply. I don't know about other but I will not sell my account even if someone offer me to pay 1 Eth
Selling an bitcointalk account may be a money making oppurtity for the account farmers or even geuine seller who want to leave the forum forever, but the buyer who paid for that account will always be at the risk. The reason for this is becasue once the accoutn change hands, for sure, there will be change in the posting habits and other stuff.
Since everyone wantches every closely to the accounts, sooner or later it will be detected that the account change hands and it will be tagged, making it useless to join the signature campiagns (I assune anyone would buy account for the joining the Sig. Campaiagns and no other reason comes to my mind).