EdStainless: the distributors we're using are local to us, i.e. they do business and have assets in a jurisdiction in which we can sue THEM if they fail to stand behind the materials they import. They import directly, inspect the mills, and do their own supplemental Q/A, or we don't buy from them.
Despite this, we have had issues- with seamless tubing (tubing being something you know well!)- and they were forced to stand behind the crap they sold us. In that case, it was Chinese origin seamless tubing in 316/L SS which looked perfect everywhere you could see from the outside and which had a lovely mill cert, but which was severely contaminated and/or corroded in the bore, the closer you got to the middle of each length. If we buy Chinese seamless tubing in future, which is doubtful due to that experience, we will need to cut a representative number of lengths mid-length and inspect the bores prior to acceptance- because that's something the distributor just isn't going to do for us.
gtaw is correct that the customer of anything has every right to exclude any vendor for whatever reason they see fit. They are under no obligation to accept the lowest bid proposal, or any proposal for that matter. They can select or exclude based on any criteria they see fit, fair or unfair. It's a different matter for public tenders.
The process of buying something, just like the process of hiring someone, is fundamentally discriminatory, i.e. you are discriminating between vendors or products or candidates to buy or hire the one you want. The key is to ensure that this discrimination is made on bases that actually matter to quality or performance, rather than on bases which are easier to quantify but which have little to no real bearing on quality or performance. Doing the latter isn't just wrong from a moral perspective, it's also a bad business decision.
Many people use brand name or country of origin as a proxy for a proper quality inspection system. A better system is an approved manufacturer's list, where mfgs of a particular product are vetted against certain quality criteria before they go on the list- and the criteria are established such that new vendors can be put on the list if they meet the required standards.