Is it permissible to sell short-sleeved t-shirts to young people, including boys and girls, knowing that the intention is to sell them to boys only, but girls might also buy them?

If you predominantly believe that girls use these clothes inside their homes, then your sale to them is permissible. However, if you predominantly believe that they wear them in front of non-mahram men, and you are unable to refrain from selling, then it is not permissible for you to sell them these clothes. It also appears that it is not permissible for you to exclusively choose the youth demographic with the intention of selling only to males in this situation.