Is the item in its original packaging with the tag still attached, or has it already been opened?
Because that's the key thing here - lots of marketplaces and sellers protect themselves by adding exclusions for clothing and footwear to their terms! By law you really can return an item of proper quality within 14 days, but some categories might be excluded from that right. You need to check the specific return conditions on the platform and with that particular seller - sometimes special rules apply to underwear, clothing, and shoes!
If the return is formally allowed though, the main thing is proving you actually haven't worn it! No signs of wear, no defects, no stains, no wrinkles. The tag should be intact, the packaging in good condition. If everything's clean, then the seller's complaint is unfounded, and you can file a complaint with the marketplace. Usually the platform sides with the buyer in cases like that!