Rule #1: Used doesn't count. Someone always has "great deal" or something else, that no one can realistically replicate. Yes, it's fine, just getting a $5k bike that's a year old for $1500 isn't relevant.
That said, it should be relatively easy to build a decently light bike for <$2k. Open mold frame/fork ($800, 1200g). SRAM Force (lighter 2.2kg), or Ultegra ($500). Vuelta wheels from Nashbar ($200), 1580g). That leaves roughly $500 and you're at a base weight of an ~5kg, to finish things off with bars, stem, saddle, tires.