buyer (n.) a person who acquires a possession, ownership, or rights to the use or service of by payment especially of money
seller (n.) one who offers something for sale
agreement (n.) an arrangement between parties
negotiate (v.) to confer with another or others in order to come to terms or reach an agreement
negotiation (n.) the act or process of negotiating: successful negotiation of a contact, entered into labor negotiations
negotiator (n.) a person who negotiates