I gave my friend a baby stroller that I don’t use anymore because she really needed a new stroller and couldn’t afford one. Two months later, we’re talking and she casually tells me that the stroller wasn’t really working for her, so she sold it on Craig’s List for $200. Don’t you think she should have asked me if I wanted it back before she sold it? I’m really pissed.
The first thing I thought when I read your question was, “What is wrong with people?” I ask myself this question about six thousand times a week, like when I’m getting a sandwich and the sandwich guy scratches his crotch. Fine, he’s wearing gloves, but his gloves touched his crotch and then they touched my sandwich, so really, it’s like he just put my sandwich on his crotch. What’s wrong with him?
The tricky thing about your situation is that you gave your friend the stroller; you didn’t loan it. Once you give something away, you really need to let go of it. It would have been more than appropriate, well-mannered, and nice of your friend to call you up and say, “That stroller isn’t really working for me. Would you like it back?” This gives you the opportunity to say no, sell it or get rid of it, or say yes, please, I would like it back. Selling it and then calling you and gloating about it is at the very least numb-skulled, and at the very most rude and weirdly braggy. At the very least it would have been nice for her to take you out to lunch with the money. But is it wrong enough for you to confront her over it? No.
If you want to get back at her, though, you could always tell all of your mutual acquaintances about it, but pretend that you think it was a great idea on her part. Not that I condone talking about people behind their backs.