Is informing my friend that an unknown person laughed at his merchandise and called it "a toy"—without identifying the person—considered backbiting, especially after my friend guessed the person's identity and I denied it?

Talebearing is not considered the transmission of someone's words if the speaker's name is not mentioned, because talebearing is the conveying of speech to cause mischief and to uncover what one dislikes to be revealed. Similarly, backbiting is not considered backbiting as long as the person is unknown, for there is no backbiting except concerning a known person.