Can I use a texture I buy here in a theme that I sell?
Yes, this is permitted so long as the item you create is an End Product. End Products must be significantly different than the original Item and require time, effort, and skill to produce. End Products must not be used or sold in a way that is directly competitive with the original Item you purchased. End Products must not redistribute the original Item to any third parties in a manner that allows for the extraction of the original Item.