This is done all the time. You have countless options ranging from selling their unbranded items at a very low cost to working with them to improve an existing design and selling it under your own brand. There is nothing wrong with slapping your own brand on it and selling it as is; just be aware you are entering a very competitive marketplace. You'll want a unique selling position, be it price or design.
Make sure you adhere to labeling requirements when selling textiles.