I mean we have Truma and kadshim where we know words in the heart are words and we have an open Gemara in Shavuot circa 26 that says we don't learn from them because of the rule שני כתובים הבאים כאחד אין מלמדים
Marriage is an act so we clearly need words. Buying and selling are acts so we need words to show intention.
See Kidushin 49b and the Rashba there , and Ketubot 75 and the Tosphot there.
Rambam Hilchot Shavuot 3 about an oath to a person that is using violence.
And what about Chametz and הפקר letting go and abandoning something. מבטלו בלבו ודיו he nullifies the Chametz in his heart and that is that." What about דברים שבלב אינם דברים