INFO: did you make it clear to her that you liked the trousers yourself, but would give them to her as you thought she would get more use and enjoyment out of them, or was that only something you thought to yourself?

If she accepted them knowing this, then she's going against the spirit in which the gift was given by trying to sell them so soon and you wouldn't be TA for at least asking for them back if she doesn't plan to wear them.

If you didn't say anything, then even though it's tactless of her to sell them soon after, you would be better off just letting it go as she wasn't accepting them with the understanding that you were giving them to her so she could get enjoyment out of them than you probably would. You just gave them to her.

Either way, she is under no obligation to give them back.