The seller should be able to set their own conditions of sale.

Unless there is something wrong with the item or the item is mis-described, if the seller says No Return then it is no return. If the seller advertises a piece of clothing as M, for example, then the buyer should be careful. "Medium" is a loose word, it may or may not fit the buyer. The buyer is free to ask for the measurements in cm, but if the buyer buys without any question and it does not fit then the buyer has no one to blame but himself.

I have had a few complaints about my items being too small. In my mostly-unprejudiced opinion, if the listing did not state the size and you did not know the size when you bought it, then you void the right to complain about size. Heck, buyers complain about sizes anyway whether or not it is stated. The worst case I've had was a buyer who bought an item with "14cm Large Keepsake Urn" in the title, the very first line of the description in bold and bigger font, and the bottom of the description. The next thing I knew she opened a SNAD case - "I bought a large urn, I received a miniature item". I sell a handful of 14cm large keepsake urns and over 80 normal sized 7cm keepsake urns. Even if I did not have "normal" keepsake urns "14cm" was repeated 3 times. Even if "14cm" was not mentioned the word "keepsake" should have been a clue. Needless to say I won that case.