I have found that whenever I courteously (and VERY politely) message a given vendor on the DNMs to complain about product quality before leaving feedback, they practically never act professionally. They get super defensive and start attacking me.
It's like, your profile says you want me to message you first... so I do. Why are you throwing a fit? It makes me just want to leave feedback without consultation from now on. I'm tired of having to qualify the fact that, hey, your product isn't the best and I am not the only one who feels this way.
Full disclosure, I do not make a habit of testing my material with reagents. I understand this is blasphemy in the harm reduction community, but I think it's still pretty obvious when a drug is not up to snuff.
/rant
A lot of vendors want you to message them if you have a problem before you leave negative feedback/dispute. Some will try to resolve the issue, while others will try to use this as a last shot to bullshit themselves out of any responsibility to you. I'm weary of vendors who use this term because of the latter vendor types.
I usually message first if I have a problem anyways, but if I feel they're bullshitting, or they don't respond, I will go ahead and file a dispute. At the end of the day, a vendor is not allowed to make up his/her own rules on selling to bypass the point of escrow.