Do as you please, but I would not sell those. Set Karma aside, if your CEO finds out you took his generosity and turned it to profit, I suspect he won't react well. (I suppose there's a small chance he'll promote you for your capitalistic spirit, but my guess is he'll get bent you made a profit on items he intended people to use.
I made sure I asked his secretary if he was okay with it, she's the person who gave them to me to sell for her (splitting the $ with her). From what I hear, he's done it in the past and the shirts just sit there forever and nobody takes them. There might be 4 people in the office that would recognize those brands, myself and him included and the others are very well-off financially.