Honestly, I don't know why you're being down voted. I feel like this scenario is a damned if you do, damned if you don't issue. Ok, she's been wanting to sell it for months. I'm assuming this guy is the first guy how's interested? She put your number on the for sale sign. So, if you had to make him wait to call her then you probably would have lost him. So, she'd probably get mad at you for losing a buyer to ask her for permission after she put your number there.

If I put someone's number on the ad, I'm giving them full responsibility for taking care of this business. Don't call me on my vacation and possibly lose a buyer. Why else would I give them your number? Of course, it's probably because she can't take international calls. But then if I wanted final approval, I'd just say so. Like, "hey I'm putting your number down but call me first before selling."

Anyway, I think what most commenter are saying are right in that you didn't communicate with her and that's possibly why shes mad. At the same time, I think it was mostly her fault in that she didn't communicate what exactly she wanted either.

So, in the future I would just make sure that there aren't any open ends as far as communication goes in your relationship.