If you buy from a business you have the right to return unused, non-custom, items for any reason under distance selling regulations.
You do not have the right to expect the seller to pay any of your return shipping costs.
Sellers on Amazon, eBay, Etsy, or wherever, and those running their own online selling operations directly, are free to offer free returns if they want to. The platform is irrelevant to that decision, but some may provide ways to make the return process very easy.
If you're returning something because you didn't like it, or it didn't fit, or you just changed your mind, and you're asked to pay return postage, or print your own labels, or include an intimate photo, that is entirely the decision of the seller, and has nothing to do with the platform.
If you're returning something because it is faulty then the seller has to bear the cost of that, no matter what they may try and claim in their terms & conditions.