Girlfriend doesn't want me to sell my laptop?

So my girlfriend's mom bought a lenovo yoga 2 11. She hated it and was going to toss it out, mainly because it stopped working. I work on computers for a living so my girlfriend's mom instead just gave it to me to do what I wanted with it and she got a different one. I checked it out and the only thing that was wrong with it was the hard drive. I swapped it out with a new drive and now i have an offer of $380.00 for it. I told the girlfriend and she's mad that I'm trying to sell it. I don't even get why? It only has an 11 inch screen and the speakers suck so i can't really hear a movie on it or anything without headphones. She has her own laptop so that's not why. What's the deal?

Added (1). I also have a high end desktop I built and don't really have use for a laptop, I'm also pretty broke and could really use the dough.

It was given to you, therefore it's yours. Maybe you could give your GFs mother a small amount of money just to show your appreciation. But really this has nothing to do your Girlfriend.

Because it was her mom's, and/or she sees it as a gift? Why not ask her?