When I started collecting back in 2006, I remember standing in the diecast isle and just staring my mouth open at the dozens of pegs full of HW not knowing what to look for.
So I decided JUST to complete the First Edition cars.......I just wanted to complete that series......I think I cried....
Even if you want to complete one sectional line, it's almost impossible to do so. I remember later doing an inventory and taking out all the 2006 FEs and having way more than just the number of cars in the line. AND I was missing some!
As PY said, unless you have contacts, it's very hard! And jhc has the right equation for it!