Friend being a royal snot?

Last semester I asked my friend , lets call her B, from high school to move into my dorm with me. My previous roommate had her father make our wooden lofts for our beds. B bought the loft from old roomie..don't remember for how much. Now I'm leaving that school, and my roomie is getting a new roomie, lets call her S. I offered to sell my loft to S for $40, which I think is quite reasonable considering it is of very high quality and was only used for a year. A couple of days ago, I receive an e-mail from S saying that she and B had talked, and they both think that $40 is a little steep and would like to buy it for $20. So whatever, I sold it to her for $20. This downright pissed me off though, that B, my friend, would go behind my back and discuss something that S and I had already established. And now B can't understand why I'm upset with her. I don't know what I should tell her? Advice please?