Next Level Purchasing...
Next Level Purchasing has had several good articles on how to stop ''back door selling''.
First and foremost, I would go with these 3 main points;
1: Engineers typically have very little negotiation skills. They get their eyes focused on one particular vendor/product, and sales people are trained to recognize this focus. Your organization loses negotiating leverage when it is apparent the only thing the engineer wants is one particular product. Beyond that....Purchasing people are trained (or experienced) in the multiple avenues of negotiation, and when to apply them. Engineers? Typically not.
2: Engineers typically focus on a narrow set of criteria. Payment Terms? No. Liability? No. Indemnification? no. Insurance requirements (with additionally insured?) No. Frankly put, when an engineer focuses on the widget and the dollar figure, they're leaving out a tremendous amount of consideration for all the other stuff that matters as much (if not more) than the cost of the widget.
3: Engineers typically respond with "but we've been dealing with Acme Widget Manufacturers for 20 years, and so they become straight to me, because we have a "relationship". True, relationships DO matter in the B2B world. No one is denying that. However, we, as purchasing people, should always remind our fellow employees that suppliers/vendors are there for one reason, and one reason only; to make money. From us. That's their job. We provide the cash, they provide the product. and they're making a designated percentage of profit off of the product they sell to us. When purchasing is bypassed (and many sales people are trained specifically to backdoor sell), naturally, profit margins increase. Is your engineer ready to explain to their CFO/CEO that their costs are higher because the engineer was backdoor sold? Probably not.
When all else fails (trust me, some engineers are this stubborn), ask the engineer to name a single Fortune 500 company that allows backdoor selling. From your mom-and-pop to wal-mart, purchasing purchases, and engineers engineers.