i bought an rc car without knowing all the buttons, i thought i can configure it out, because the seller has demonstrated on its used, i didn't saw the buttons fully, but i thought it does some features, apparently, there were extra buttons that doesn't do anything,

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

The mere fact that some buttons do not work does not necessitate that this be considered a fault or defect, for they may be buttons for additional functions not supported in this model.

Generally speaking: the criterion for a defect is whatever normally causes a decrease in the value of the sold item, or defeats a legitimate purpose for which it was purchased. If the buyer finds the sold item defective, he has the option to return the item.

It states in Al-Mawsū'ah Al-Fiqhiyyah Al-Kuwaytiyyah: "The criterion for a defect in the sold item according to the Hanafis and Hanbalis is: whatever causes a decrease in the price according to the custom of traders, because the harm lies in the decrease of monetary worth, and that is through a decrease in value."

According to the Shafi'is: “it is anything that diminishes the item or its value in a way that defeats a legitimate purpose, if its absence is predominant in that category of goods, whether it coincided with the contract or occurred after it, before possession.”

According to the Malikis: “it is the existence of a deficiency in the sold item or the price, when the custom is soundness from it." [End of quote]

Ibn Qudamah said in Al-Mughni: "Whenever he – i.e., the buyer – becomes aware of a defect in the sold item that he was not aware of, he has the option between keeping it and rescinding [the sale], whether the seller knew of the defect and concealed it, or did not know. We know of no disagreement among the people of knowledge on this." [End of quote]

Allah Knows best