Is the account yet in a signature campaign? I bet if the account has a terrible post quality it would get accepted anywhere.If someone is buying the account to earn from signature campaign,then this account would not be a lucrative option for him since he has to put in effort of posting either 100-150 posts of GREAT quality to have him accepted .Which make the value of your account to 0.05 to 0.06 .Usually you get a good potential senior member for 0.10.
It has never been in any sig campaign and honestly i don't know why it gives that post quality.. there are maximum 5 posts in selling section.
Doesn't matter,the overall post quality is counted in the price estimators.Trust me,the algorithm for judging the quality of posts doesn't judge by its actual quality but only quality.If you have a number of posts which are short and maximum5-10 words,it will be counted as bad post regardless of how constructive it was in that particular topic.Irony is,even sig campaigns don't like people with short posts .