Is it permissible to sell T-shirts designed for men and women together as unisex?

The basic principle is the permissibility of selling anything that can be used in a lawful manner, unless it is known or strongly presumed, based on circumstantial evidence, that the buyer will use it in a forbidden manner. T-shirts fall under this principle; both genders can use them in a lawful way, so selling them is permissible, except in the case mentioned.