On the over-the-counter market, parties agree on terms independently, without centralized order matching. Securities that have not passed listing, large packages whose sale on an exchange would move the price, as well as instruments with individual parameters are traded here.
The main difference from an exchange is transparency and guarantees. An exchange publishes quotes and ensures transaction execution under uniform rules, while on the over-the-counter market the price is known only to the parties, and the risk of counterparty default falls on the participants.
For a private investor, the practical conclusion is this: the price of an over-the-counter transaction has no external benchmark, and it is difficult to verify its fairness. If an offer to buy securities comes outside the exchange, it is worth clarifying who acts as the counterparty, how the transfer of rights will be formalized in the depository, and why the transaction is not conducted through organized trading.