Other sources of interest in this matter include Pitchei Teshuva (Yoreh De’ah 364 ad loc.), which quotes Birkei Yosef, who is very strict with regard to selling the original stone. Mahari Navon rules leniently in principle, but rules stringently in practice, forbidding the sale of the original stone. The Chatam Sofer compares monuments at graves to cemetery walls and permits using them for other communal needs.
We learn from Rabbi Feinstein that local custom is crucial, both with regard to erecting gravestones and with regard to the permissibility of reusing older stones. Therefore, if a person is dealing with a particular situation, he should consult rabbinic authorities in his locality.