Before us is a Mishnah in Maseches Gittin, chapter 5, which deals with the valid means of communication available to a person who is deaf and mute, and with the validity of purchases and sales carried out by him.
A cheresh - "romez v'nirmaz":
The Mishnah rules that a cheresh "romez v'nirmaz" - he communicates by means of his hands and his head, a kind of sign language, and in this way he is able to express and indicate his wishes: what he wants to buy and what he wants to sell. These signs are considered a clear expression of his intent, and therefore the transaction takes effect.
The opinion of Ben Beseira - "kofetz v'nikpatz":
Ben Beseira says "kofetz v'nikpatz" - there is no need to rely specifically on signaling with the hands and the head; one may rely even on the way he expresses the matter with his mouth, by moving his lips. Even though this is not as clear a sign as signaling with the hands or head, it is sufficient. Ben Beseira, however, limited this:
With movable property: one may rely on "kofetz v'nikpatz," that is, on the movement of the lips and the forming of the words with the mouth.
With land: Ben Beseira agrees with the first Tanna that one must rely on "romez v'nirmaz" alone.
There are commentators, however, who understood that the word "b'mitaltelin" (with movable property) in the Mishnah, which limits the ruling to movable items, was stated even according to the first Tanna - he too said his ruling only with regard to movable property.
Transactions of minors:
The Mishnah continues and discusses pe'utos, young children who have not yet reached the age of mitzvos. The measure of their age is determined according to how sharp they are:
Sharp and quick to grasp: from the age of seven or eight.
Not so sharp and understanding: from the age of nine or ten.
Concerning them the Mishnah rules: "mikchan mekach u'mimkaran mimkar b'mitaltelin" - their purchases and their sales are valid when the matter involves movable items.
The Gemara explains that this validity stems from an enactment of the Sages who permitted them to carry out transactions of this kind, "mishum kedei chayav" - so that they be able to sustain themselves. For at times they are not under the care of parents or under any proper supervision, and they must make their own way in the world, and for that they need the ability to conduct transactions.
In summary: in this Mishnah we learned the ways in which a cheresh reveals his intent in buying and selling - "romez v'nirmaz" according to the first Tanna, and "kofetz v'nikpatz" with movable property according to Ben Beseira - as well as the validity of the transactions of minors with movable property, which was established by an enactment of the Sages so that they be able to sustain themselves.