(Note: I am a guy, and therefore many girl posters may consider me insensitive for what I am about to say. But in real life I'm very sensitive.)
You are definitely not an asshole for selling off that present on eBay. Whether you are for noding about it depends on whether you get caught!
Seriously, though, it's like this: the gift was given out of love, right? Out of a sense that maybe this was something you would like and find useful. And, well, you would get more use out of something else, so you decided to sell it. Admittedly, it would have been courteous to let her know that you intended to sell it for something more appropriate to your needs, and that you still love her. But she should understand that anyway. She should also understand that you are much more likely to remember her gesture and the occasion if you have something you actually use than if you have something forgotten in a drawer.
She sounds pretty controlling. In fact, if she is taking issue with it, it sounds like she was trying to buy your love or manipulate you with it:
You can do what I want you to do, or nothing at all. But don't do something that makes you happy with what I have given you.
Now that you are selling it she feels less in control. That sucks for her, but she didn't give it in the right spirit. That kinda sucks for you, because you found this out, but now you know what spirit she gave it in, and you can think about the relationship.
One final note: If she claims that you don't appreciate her effort in trying to find something for you, that pretty much proves what I said: she is trying to buy your love. You will know this for sure if she tries to break up and says anything about presents either of you have given each other, either that she wants to trade them back, or conversely if she makes a point of saying you don't have to. If they are tokens of love, freely given, then they shouldn't come up. If she does, she's pulling a Cartman.