Should the buyer demand that those who signed the bill of sale defend the minors, or what is the solution in this case, given that there are minors who did not sign the sale document, nor did their guardian, and the buyer has built a house on the land without anyone objecting?

The issue of selling the property of minors depends on the existence of a necessity or an apparent benefit, such as the minor needing maintenance or selling a property for more than its value to buy a better alternative. The consent of the adult parties is required for the sale to be valid. As for the sale by an unauthorized agent ( فضولي ) (one partner selling the share of another partner without authorization or guardianship), it is a matter of dispute among jurists. The Hanafis, Malikis, and one narration from Ahmad hold that it is permissible, and its enforceability depends on the owner's ratification. In contrast, the Shafi'i and another narration from Ahmad hold that it is invalid. The sale by an unauthorized agent is not valid if the owner is not eligible to ratify it, such as a minor.