Hey, I won’t recommend you used backdrops, as very often their quality will be too poor. Also, you cannot be sure of patches and marks that can badly affect the quality if your pictures. There are many stores that sell discounted backdrops. They are a better option than buying the used ones.
One store that I personally know and have used their services is PhotoStudioSupplies. Their backdrops are quite good, especially the green screen ones. I had their complete green screen kit sometime back and I am still using the backdrop that was there in the kit. It has an anti-reflection surface and is made of 100% muslin. It’s been more than 2 years and it’s still in a good condition. They keep offering further discounts time to time. You may also try any other store but PhotoStudioSupplies is quite trustworthy.