In Aristotle's classification, all interpersonal acts (synallagmata) are divided into voluntary and involuntary. To the first category belong: buying and selling; lending money; pledge; loan on condition of safe return; deposit on trust; and hire.

Selling took place with an immediate exchange payment, and hardly ever on credit. There was, according to one ancient testimony, a law of Pittacus forbidding credit. If one of the contracting parties was unable to at once hand over either the object being sold or its equivalent value, an arrhabon, a kind of communalized preliminary contract, was entered into. This contract was binding, and transgression of it entailed prosecution at law. The arrhabon is, however, known only from classical sources, and it is not certain whether it applied in the Archaic period. One of the seller's obligations was to make clear (prolegein) to the buyer any imperfections in the object being sold.

One of the more ancient formal contracts in Hellenic law was the loan. It has been argued that for a loan to be valid, it was essential for witneses to be present; it appears, however, that - as in most cases - the witnesses obviously played only a probative role. We have more details from the Classical period, in connection with the separate category of the maritime loans (chremata nautika), for instance. Their special features were: accepting of risk; concluding of exceptionally high interest rates; pledging; and the corresponding written document. Let us not forget that at the time we are talking about, shipping was an undertaking with many perils (weather, pirates, hostile natives close to the ports of destination). For this reason, maritime interest rates for every voyage could be in excess of 30%, whereas interest rates on land (eggeioi tokoi) were not above 12% per annum. Differences arising from such contracts were heard in naval and commercial courts. Lastly, there was taken as surety (hypotheke) either the vessel itself, if it was the captain who was taking out the loan; or part of the cargo, if it was the trader chartering the vessel who was doing so.

The roots of the associations called koinoniai chrematon can be traced to kinship and religious forms of organization, such as the syssitoi and orgeones. Solon gave them the right to draw up their own constitution, for which simply the endorsement of the members was required. A sine qua non for it to be in force was that it did not go against the standing laws. From their foundation onwards, the associations acquired competence in law for all acts that did not require the status of a natural entity. They also had competence as contractors (ekleptoria) vis-a-vis the city, in mining and trading operations and in other public works.