There is no way to know for sure, unless a different writing style, new language is spoken or good english gone bad, but all of these are not 100% solid proofs. However the answer to this question is not very important because ,

In most cases account sellers do not publicly post the account they want to sell, they will use an alt to advertise it, then the negotioans will be done outside of the forum.

One need to be pro-stupid to advertise his account using his account, it will be tagged instantly and nobody would want to buy it.