If you are responsible for buying all the candy for a school sale, you are responsible for purchasing the candy. To make the job easier, you might start a purchasing committee. Purchasing is buying.
The word purchasing sounds more official than buying, but it means the same thing — acquiring something by paying for it. In larger businesses, the purchasing department or just "purchasing" is an entire section of the company dedicated to doing the buying. If you lead this department, you might joke that your job is Top Shopper.
