Stores require the clothes returned are in sellable condition, so if you return worn clothing (with or without tags if that's required) then is it a sin?
Yeah it would be tacky and silly, I know, but actually a sin?
I cannot say it is sin or not sin because I am not qualified to make a fatwa.
But I have a story. One day Rasulullah (saw) visited a market, and he saw a vegetables trader mixed fresh and 'not fresh' vegetables and sold those as fresh vegetables. Then Rasulullah (saw) told him not to mix those vegetables but he must share those into two categories, fresh and not fresh, and sell those in different prices.
One rule in sharia trading is honesty.
When you bought clothes from the store, you bought it as new clothes. But after you wore that clothes (wearing, not fitting), that clothes has turned into used clothes. Not new again.
When you buy clothes, the store gives that clothes to you and you give money to the store. This is a trading, you are the buyer and the store is the seller.
And when you return that clothes, actually this is also considered as trading, because in this case, you give that clothes to the store and you get money from the store. You become a seller and the store becomes a buyer.
Of course, you cannot sell used clothes as new clothes.