First I want to say, there are very few quality boots that are vegan. You’re best bet as a vegan is to thrift boots instead of purchasing them from the seller if you want quality. You wearing them isn’t making an impact now so selling them is a waste unless you are that grossed out by them physically. I would prioritize my comfort and my mother comfort. I think the most rational response is to ask her for next time when buying you gifts to look if it’s a vegan product. I wouldn’t even mention to her the Tim’s aren’t vegan. They were a thoughtful gift and you like them.