From what I understand, having Resale Rights means that you can sell that product yourself. Having Master Resale Rights means that you can sell Resale Rights to that product, in addition to selling that product outright.
Whether or not you can make changes to the actual product should be covered in the sales copy or FAQ for each individual product. Obviously you can alter or use the product/content in any way you choose if it is content - but other than that, it's more of a case-by-case basis type of thing.