Not to state the obvious; but the old black pads on those calipers you got off ebay or at a flea market might very well be the pads that came with those calipers from new. No surprise that a fresh set of Koolstops or Mathauser's are like night and day first time you try them.
btw: the grey Campy replacement pads for Victory(?) calipers work good too.
Pete Geurds
Douglassville, Pa