Don’t create bidding wars with vendors. “What kind of deal can you give me on these tips? This other vendor is giving it to me at a 5% discount per case.” I know. I know it’s fun. Especially, when you actually get the discount. First of all, I’m not saying bidding wars don’t work, but knowing when to do them matters. Medium to large capital equipment (lets say…$1000 and up), costly experimental reagents, or bulk orders may be the best times to apply this age old strategy. But when you use this tactic for nickel and dimming your local vendors, it spoils an important commodity in academic research; relationships.

You end up being known as the penny pinching lab and it ends up not being worth their time or discounts to work with you. You develop a reputation that you didn’t intend to and you end up loosing more in potential future discounts.
I suggest before working with a vendor, focus your purchase analysis in the following order: need, quality, then cost. First, know what you need or want in the product.  Next, compare quality of the products. Then, check the prices. If you find that 2 companies have the same quality product for your needs, at a comparable price, then work with the vendor rep that you have the best rapport.
Offer suggestions that will help them improve their product or services to help you meet the lab’s needs. It lessens the time they have to spend guessing at what you want. Manage the relationship by keeping them up to date with future changes you may be making.
Consistency of a good relationship will pay off when they offer discounts down the road.  They can also pass on other discounts from products of other vendors that they, themselves, have developed a close relationships with.  “I like this lab. They’re easy to work with, give them the best deal you have.”  Ka-Ching! Be a good stewart of the sciences. Vendors are part of community, too.