Is it permissible to sell land that has been acquired by placing stones and wooden pillars in it, before or after cultivating it? And is building upon it considered a form of acquisition?

It is permissible, according to Islamic law, to lay claim to and develop barren land (ard mawāt) that is not owned by anyone, based on the saying of the Prophet, peace and blessings be upon him: "Whoever revives a barren land that no Muslim has a right to, then it is his." And also his saying: "Whoever is first to a permissible thing has the greatest right to it."

A condition for owning land through development (ihyā’) is that it must be free from any prior claims or protected ownership. If what you did constitutes development according to custom, then you are permitted to sell it and dispose of it. However, if it was merely placing wood or stones, which is not considered development according to custom, then it is not permissible to sell it because you do not own it. But if your claim to it grants you priority over others in benefiting from it, then there is no harm in relinquishing it to another person in exchange for compensation. This is known as "al-isqāṭ" (relinquishment) or "al-farāgh" (vacation).