Every now and then you run into a shop or a dealer at a show with a basket full of brown-paper grab bags. I usually assume that these are full of commons and nothing else, but last summer, as I was visiting my parents in Fargo, ND, I went into the LCS with a few extra bucks and decided to give a couple a try. The hockey bags gave me a bunch of Victory leftovers and basketball gave me a bunch of chromies, but football was the big winner. I didn't know it at the time, but I had gotten quite the card out of a bag full of serial-numbered goodies. Keep in mind, this was before the 2010 football season, so nobody could have predicted that this guy would soon lead the league in rushing, break team rushing records and earn his first trip to the Pro Bowl. Even better, it is a die-cut rookie card with a low serial number of 21/27.
While, yes, most of the time this is a way for dealers to rid themselves of commons and other unwanted junk, every now and then, you find yourself a gem.