I would not recommend used or relay ties. Not only is the cost of installation no better than with new, you always have some cost in getting the ties to the jobsite and usually you have to load them and ship them from the old line they came out of, distribute them along the line where they are going to be installed and then plug the old spike holes (if you really wanted to get as much life as possible out of them). Now, if one can get a lot of this accomplished through volunteer efforts, it might make sense, but in the end I must encourage the purchase of new ties.