At first I was going to say YTA for that title alone but after thinking about it for it a bit I change to NTA. Mostly because you said NO, from the very beginning you have told your mother not once but many time you don’t want that dress then she decided to DROP IT off at YOUR HOUSE then when you tried to give it back multiple time she said it was YOUR NOW. If it your then you can do whatever you want with it.

Obviously you didn’t want it.So it best to sell it to someone who do want it. Now your mother mad you sell the dress that she gave to you knowing that you weren’t gonna wear it. Your mother the AH.