Dear Brother,
Greetings,
The answer:
Abu Dawud narrated by way of Aisha (ra):
كَدَسَّهُ فِي جَانِبِ الْقَبْرِ كَسْرُ عَظْمِ الْمَيِّتِ كَكَسْرِهِ حَيّاً "The Messenger of Allah (saw) said: 'Breaking a deceased person's bone is like breaking it while they are alive.'" (Narrated by Abu Dawud)
It is mentioned in Awn al-Ma'bud Sharh Sunan Abi Dawud:
Al-Suyuti stated in explaining the reason (sabab) for the Hadith, on the authority of Jabir (ra) who said: We went out with the Messenger of Allah (saw) for a funeral. The Prophet (saw) sat at the edge of the grave and we sat with him. The gravedigger brought out a bone—a shin or an upper arm—and went to break it. The Prophet (saw) said:
لَا تَكْسِرْهَا فَإِنَّ كَسْرَكَ إِيَّاهُ مَيِّتاً كَكَسْرِكَ إِيَّاهُ حَيّاً، لَكِنْ دُسَّهُ فِي جَانِبِ الْقَبْرِ "Do not break it, for breaking it while dead is like breaking it while alive, but rather tuck it into the side of the grave."
This means that if the bone exists, it must be protected and kept in the soil.
Accordingly, exhuming the graves of Muslims is not permissible unless the bones have decayed and become like dust. At that point, it is permissible to plant there, build upon it, or perform other mubah (permissible) actions.
However, if the bones still exist, it is not permissible to exhume the graves, build on them, or perform any other actions except in special cases related to the deceased or similar circumstances (as clarified by the texts).
As for how much time is required to know whether the deceased has decayed or not, this is referred to the experts and the tahqiq al-manat (verification of the reality).
Regarding the cemetery you mentioned, which you say is 500 years old and that nothing remains of it because it was previously bulldozed, the traces of the graves within it vanished, and a commercial market was built upon it: the sin lies upon those who bulldozed it if there were bones present at that time. As for the current situation where the cemetery has disappeared and no traces of it remain, as mentioned in the question, and the market was built on the land after nothing was left of the cemetery, then buying and selling in this market is not haram.
The sin is on the first person who bulldozed it and removed its contents if there were bones present when he removed them.
Accept my greetings.
11/07/2004 CE
Your Brother