Baba Basra 154a - How does simanim indidcate that he was an adult when he sold?

The gemara quotes a story in which there was a dispute between the inheritors and buyer whether they should dig up the deceased to see if he was an adult, which would verify that he had the right to sell (re: nivul hameis - see previous post). The assumption is that if they would find valid simanim, that would prove that he was an adult at the time that he sold. Why? Wouldn't it only prove that he was an adult at the time that he died, but perhaps at the time he sold he was still a kattan?

The Ketzos HaChoshen (235:5) quotes a machlokes rishonim. The Rosh in a teshuva (33) writes that if we would find that he is a gadol at the time that he died, we would be machzik him as a gadol from the time that he reached the age of gadlus (even the inheritors agree that he became 13 years old prior to the sale). But, the Ramban writes that we are speaking in a case where he died immediately after selling, therefore if he was a gadol at the time he died, he was a gadol at the time he sold.