Basically, home builders, suppliers, etc. can be members and receive benefits for their membership.https://dallasbuilders.com/

I never provide receipts for materials , what my arrangements / discounts at the supplier are none of the customers business .  

“Higher valued commercial real estate exchanges are an important source of jobs for contractors, skilled and unskilled blue-collar workers, lenders, real estate brokers, Qualified Intermediaries, title insurers, escrow companies, surveyors, appraisers, architects, landscapers, building material suppliers and more.

Short of you personally paying all the suppliers and sub contractors its next to impossible to protect yourself if the GC is going to go wonky on you.. 

So if you need an extra bundle or so, you can go there instead of trekking to a supplier to get it.True, type/brand of "underlayment" doesn't matter so much, like ice/water or a good membrane to put in the valleys (over the valley metal), and if they are doing the Ice/Water around the perimeters, THEN it may matter for a good one.Flashings?

Others will say " dont give deposits " , "You buy materials "   , or " the contractor should use their accounts at suppliers for materials "       WRONG , The contractor is NOT there to finance your improvement , thats the owners responsibility .

If you were a builder with a dedicated crew, discount suppliers, and local lots in the pipeline, then yes.

Anyone have some suggestions on wholesalers or suppliers for quality and fairly priced Kitchen and Bathroom necessities; flooring, cabinets, backsplash, sinks besides Home Depot and Lowes.

Tesla suppliers are looking to build nearby. 

For instance, in NJ, our utilities supplier offers payment plans for a new HVAC system.