Bad Idea #22 - Forgetting that it takes two to agree on a transaction

My roommate is getting rid of all his books, and he asks me if I want to read any of them. I ask about a few, and only one sounded interesting to me. I tell him I'm moderately interested in that one he mentioned, and then forget about the conversation.

The next day, I come back home to the book sitting on my desk. I think, cool, I'll check it out when I have time. The roommate peeks his head out and says

"Hey, it's sitting on your desk. I think you'd enjoy it."
"Cool, thanks."
"But the thing is...it's a textbook so..."
"Oh no worries. I don't have a problem with reading textbooks - I'm kinda a nerd like that."
"No, not that. It's just that it cost a lot, so..."
"Wait did you want me to buy it from you?"
"Uh...yeah."
"Oh well in that case, no, I'm not going to buy a book from you."

So he took it back. And I will probably never get a chance to read it. What a bitch.