Creating your OWN textures and selling (or giving away) them for someone elses product is 100% legit.  The only real restriction is you can't include use or any of the material that came with the product.

Using resources kits as a base for your work is also OK. Though you MUST credit the kit in your readme and some kits (rarely) have restrictions about much much of the original kit can be used.   

BTW - starting with freebie creation is a really good idea - it lets artists see how good your work, and you'll make many new friends. Artists love it when someone makes a nex texture set for their mesh!    

Pinky - you left the lens cap of your mind on again.