What is the difference between sales and purchasing in procurement process?

Purchasing is a subset of procurement. Purchasing generally refers simply to buying goods or services. Purchasing often includes receiving and payment as well.

Procurement involves the process of selecting vendors, establishing payment terms, strategic vetting, selection, the negotiation of contracts and actual purchasing of goods. Procurement is concerned with acquiring (procuring) all of the goods, services and work that is vital to an organization. Procurement is, essentially, the overarching or umbrella term within which purchasing can be found.