Without question do this and you gotta make it part of your return policy, not just stated in the listing. If you're gonna accept them in general, especially 30 days, you gotta do this. Used clothing can be worn and just returned or purchased without care or thought with 30 days to return. 20% will make them at least be smarter and more concerned with their purchase.
Is this (bolded) only applicable to people with stores?
I occasionally sell things and state in the listing "no returns" but based on all of the experiences I've seen related here I'm sure I'd end up taking any return that was requested if it came to that.