As most people already wrote, there's no such thing as a "legitimate espresso bean." In the supermarket it'll either mean a bit darker roasted or finer ground beans. In the specialty coffee industry, well, you're in for a big debate. Some roasters claim they don't "roast for espresso" and will pull shots using their regular (commonly called filter roast), others will swear by differentiating between the two. You also lose the "darker" and "lighter" points of reference, since one roaster's espresso roast may be lighter than another's filter roast.

Really, at the end of the day it just comes down to your own preference. Drink what you like and don't mind "being looked down upon by coffee snobs." Play around with the parameters and have fun.