A sale can be done by oral agreement. For instance if you buy a sandwich.
In case of the sale of real estate the sale agreemnt will be proven probably by a signed contract.
For a very official version of a contract, in Latin Law countries, the notary public will be involved in the contract. Especially for the transfer of real estate a contract before a notary public is often obligatory in these countries.
You can find out more about a sale contract on the international law forum for the following countries: