If I was taking a guess, I'd say it was an injury that caused it. Like maybe he got kicked in the teeth when he was young. It definitely doesn't appear to be recent, as the gum area where the tooth should be looks healthy and has kind of "drooped" in the place where the tooth would sit. So I'm thinking he got kicked or whatever years ago, the one tooth came out or was pulled, and since then the other tooth or teeth have died. ~But~ I'm definitely not experienced or knowledgable in this area at all!
I assumed I'd have to be concerned about dental care, but what about feeding? Since he seems to graze ok, could I assume that will continue to be the case or should I assume he should be on hay, grain, mash, cubes, or something else? I would say his weight is a little low, but since I have no history, it could possibly be attributed to lack of or poor quality feed or care.
Is there anything else I'm not thinking of? I just want to have things clear in my mind before we go forward. (A pre-purchase exam is not an option since he will go up for auction.)