You want to look for a cromoly frame.... the seller should list the steel - 4130, Reynolds 520 or Dedacciai Zero/Tre are high quality steel tubing used in road bikes. Reasonably lightweight, stiff and compliant.
If you want high end steel, Reynolds 531, 631, 753 and 853 are all good choices. There are also other steels made by Columbus, Dedacciai, True Temper and Tange that are of similar quality. They offer a great lightness to strength ratio making for a superb riding bicycle.
Good luck.