You can sell customized versions of existing content but it needs to be customized in such a way as to be a different product. You can't resell a product as is.
So for example, if there is a jacket or dress already for sale that you would like to create custom textures for, then you can sell the custom jacket or dress in the Marketplace. This kind of content is called derivative content.
Derivative content means that the customer will need to already have the base product to be able to use your custom version. If they don't have the original base product they will get a watermark.
When uploading, "derivative" content will be detected and the product page will display the required base products. You may also get warnings to let you know if a product can't be uploaded to the Marketplace and the reasons for it.