I guess this goes again back to the differences between the Middle-Earth in the Hobbit and the Middle-Earth of the Lord of the Rings. In the more fairytale-like world of the Hobbit, professional adventurers may pop from behind any corner. But in the world of the Lord of the Rings, that'd sound a bit silly. Maybe there were some mercenaries, but I think they were not very usual, and a gruff mercenary is far from an idealist jolly adventurer.
The problem, of course is, that these two worlds are the one and the same world, if we want to see this matter from the Middle-Earth, not the literary perspective. So my guess'd be that there were some of those mercenaries (which would include professional burglars) and the "heroes" would be such as (for example) Beleg in the Silmarillion or as Legolas and Gimli, professional warriors/soldiers/scouts or nobility trained in the arts of war, whose assignments happen to be adventure-like, for example steal the eggs of the giant spider and bring them to Thranduil to examine. (Okay, that was a stupid example.
) And when such person earns enough reputation in such dangerous assignments, I guess some people will start calling him or her a hero.
But to a professional seeker of some abandoned or dragon-guarded treasure I'd say: "No, you don't belong in Middle-Earth."