They buy most or thier components through the commercial surplus markets. That means that when large companies have excess or obsolete inventory, they buy it from them for pennies on the dollar. The components they sell are everybit or even better than Radio Shack. Remember the news about Cisco last year? They had obsolete inventory of over a BILLION dollars in which they either scrapped or surplused out. Companies like All Electronics, MPJA, Alltronics and MECI are the ones who buy up that inventory. Just watch out for parts they list as USED or "Removed from equipment".
I have bought most of my parts from Mouser and just like All Electronic they have done an excellent job for quality and price and service. Local sources wanted 5-10 times the money for the same item. Lew