Is it permissible to sell a piece of agricultural land – which is the questioner's and his family's source of livelihood – for the purpose of marriage, knowing the dire need for it?

If the land belongs to you and you fear falling into zina (unlawful sexual intercourse), then marriage is obligatory for you. If you cannot afford marriage except by selling the land, then sell it, because in this case, marriage takes precedence over supporting your parents. This is explained by the fact that marriage is obligatory for one who is capable of it and fears falling into hardship (sin), and it takes precedence over supporting parents because a person's expenditure on himself takes precedence over expenditure on others.

However, if the land belongs to your parents, you are not permitted to dispose of it except with their permission.