Giving gifts such as free sales promotion to a purchaser is ethical.In this case it benefits both parties.

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Giving gifts such as free sales promotion to a purchaser is ethical. In this case it benefits both parties. For a customer it may get a coupon off its favorite product or something they need or want or an added product from which can bring happiness. When a customer makes the purchase this benefits the business which brings happiness. 

By giving preferential treatment to customers who are also good suppliers. This can be defined as reciprocity, which is the exchange of materials for mutual benefit. In this case I believe it is ethical to give special treatment to customers who are good suppliers. This is creating a reliable customer base. You will  c a base that will stick with you because they know they get treated well when dealing with your business. Companies create loyal/reward programs for this reason. You as a customer feel important to this company which is vital for a company to make it as personal as they can. 

Unless the purchaser gives their permission that they can use their information, it is unethical to seek information from purchasers on competitors quotes for the purpose of submitting another quote. 

Gaining information about competitors by asking purchasers I believe is ethical. You as a business owner want to out beat your competition and in doing so you need to gain information about the other company to know how they operate, maybe even how they cut costs or spend money, and how they attract customers. It can all be very beneficial to your business. Businesses can also gain information about competitors in other ways as well. As described in the article "12 Ways to (Legally) Spy on Your Competitors", by Carol Tice, it says by just looking on social media page you can gain information about a competitor, or even play secret shopper and skip the middle man.