I've been very fortunate in my life to have been firmly planted on both sides of this fence. I am a better sales person because I WAS in purchasing. In my personal experience, the buyer and the purchaser are the same person (not literally), you want to give them what they need,and they want to buy what you have. It's how you communicate to them that makes the difference. This is only when you are dealing with the final decision maker, or the purchasing professional.
Most of the time, you as the sales person, are dealing with a product champion, a recommender, or gate keeper. They aren't so cut and dry, or open to "helping" you get through to the right people. But you can ALWAYS get them on your side with the right approach. I love calling assistants, they have all the information I need, will answer questions informally, and tell you the best approach to getting the right people lined up. And you can usually put them in a recommending position by enlisting their help to forward your information to the right people.
Sales vs. Purchasing they are both a rush. And if you are good at one, you are good at the other. It's a constant evaluation of the person sitting across the table from you.
Happy Selling.