There's a huge difference between cheap parts, and used out-of-favor cheap components. Shimano 600 hubs can be found on CL for $30 a pair with no more than 3K miles of actual use. These hubs are just as good as modern Ultegra. I recently paid $15 for a pair of near-new Tektro R538. Again, performance is par with Ultegra after spending $11 for new pads.
Centurion Ironman or Bridgestone 400 is often listed on CL for <$250, but one can often negotiate the price down to sub $150. One can spend +4K on a modern bike. However, is the potential gain of 1.5mph cruising speed worth the 26x price premium over the other used bikes?