This one is tough but I actually think both of you might have been right.
It's easy and many times it's correct to say she was wrong and she shouldn't have pushed you to sell it. But that could be hasty and perhaps unfair given the whole situation.
I take it from what I have read that you are a young man and not making a big income these days and you have financial needs that are pressing, like a new car.
I'd say as long as this was in fact a good financial decision and it's for something you actually have a greater need for, then yea, you probably did good listening to her.
Still, it hurts to let them go....... guns that is