Brian's List - interesting werbe/sondermodelle and mostly pricey (revised June 25, 2007)
Ron's List - great selection/values (revised January 24, 2007)Glenn and Sandy Stiska - Bargains (revised January 22, 2008)
Bill Seybold - Märklin Pantographs and Double Pole Reversing Switches (Revised March 24, 2008)Also, check out...
Z on eBay - (where almost everything is considered rare - by the seller)