Is it obligatory to verify the legitimacy of the seller's possession of land before purchasing it, in order to avoid sin in the event that his possession was unlawful?

What you asked about has several cases:

First: If you know or predominantly believe that the seller acquired the land through unlawful means (such as usurpation or deception), then it is not permissible for you to purchase it from him. This is because he is not its rightful owner, and purchasing it in that case would make you a partner in sin.

Second: If you know or predominantly believe that the seller acquired the land through lawful means (such as sale or inheritance), then there is no objection to you purchasing it from him.

Third: If you do not know how the seller acquired the land, and the matter is merely a doubt or an assumption, then the general principle is permissibility. However, it is good to be cautious for the sake of one's religion by avoiding purchase from someone whose matter is doubted, and to verify the lawfulness of the acquisition, especially in times of usurpation.