"Anyone, from the most clueless amateur to the best cryptographer, can create an algorithm that he himself can't break. It's not even hard. What is hard is creating an algorithm that no one else can break, even after years of analysis. And the only way to prove that is to subject the algorithm to years of analysis by the best cryptographers around." (Bruce Schneier, 1998)
You are right to be cautious of snake oil vendors. The thing is, most of them are completely sincere in their belief that the new scheme is secure. But if it hasn't been tested by people qualified to break things, it isn't secure.
The difficulty is two-fold: (1) inexperienced cryptographers are unable to test the security of an algorithm sufficiently; and (2) experienced cryptographers are not common, they are generally busy enough already, and it's tough to get them interested enough in a new algorithm to really take a close look at it.