Well, first I'd stop and ask myself whether maybe the person got two of the same item, one from me and one from somewhere else. That might be the most charitable way to look at this.
The other possibility could be that the person is having financial issues and that is behind the sale. You can't always know whether a friend is in financial straits; some people hide it very well, so assuming that "She doesn't need the money from this sale" could be wrong.
If I truly knew the person was financially fine and just an eBay addict or shopping nut, period, I would stop giving gifts of things and give a gift card to a restaurant the person likes, or would donate to a charity in the person's name, or possibly give a gift such as tickets to an event I was certain he or she would attend (and not sell).