I was reading another thread when it mentioned something that I thought would be really pertinent for you and Buster.
Fran (pinksalamander) had her bunny, Lottie, to the vet for a sore, bulgy eye and during the check the vet said she had a vitamin D deficiency. She mentioned that as Lottie had ridges on her teeth. Here is the thread where it's discussed (it's toward the end and only mentioned briefly):
http://www.rabbitsonline.net/view_topic.php?id=36084&forum_id=16
So, yes, it does seem like Buster's teeth are ridged due to an improper diet in his previous home. And, even better news is that they should grow out into healthy teeth now that he's well cared for (thanks to you)!