Generally, there are two ways to sell content in the Marketplace.
One is to create custom content that is 100% your own work.
The second way is to customise another base product which will make the content you sell derivative content. When this is done a warning will be placed on your product page to let other users know that they must have the original base product to be able to use the derivative content. The most common use of derivative content is when retexturing to give it a new look, but it can include other modifications such as reshaping.
However if what you propose is to create and sell a new mesh based off another mesh, then this falls outside of our current guidelines for submission. You can't sell a clothing mesh that is half someone else's work and half your own, even if you give credit.
So the best solution would be to use the existing product as a style guide, but then create a brand new mesh that is entirely your own work and one which doesn't include anything from the original product. You can then sell the clothing as your own work without any DRM or derivative issues.