Happened to me when I was buying a roof for my house. Went through my regular builders merchant for a price. They came back with say £7k.
I then got a quote from a timber merchant where I used to live 30 mile away. He came back with say £5k. I mentioned it to the regular merchant who put me in touch directly with their roof supplier who said he could beat the £5k price!..... WTF!!! Do you take me for some kind of an idiot!!!! No way, I'm going with the best price first time. The builders merchant rep was gutted as he lost his cut and just assumed I would purchase everything through him