In addition to what you've said, Buying is the acquisition of an object in exchange of money, whereas selling is acquiring money, in exchange of relinquishing all claims of ownership from an object. Buying and selling are two terms that are often used relatively; when ever there is talk to buying, there is talk of selling.
The process of buying and selling of goods and services is called trade. Trade is the transfer of goods and services from an individual or entity to another in exchange of money. Buy/Sell agreements are important if you own a business or even just part of a business. Without it, your business could face a world of financial and tax issues upon the owner's death, incapacitation, divorce, bankruptcy, sale, or retirement.