slander? defamation? nonsense!
Be careful not to cause yourself legal issues with slander.
Truth is the defense for defamation, and lawsuits are seldom profitable for anyone but lawyers and the courts, and I'm not talking about lies, read on...
What I favor is a registry with a voluntary annual membership – like joining the BBB for bonsai, if you will. If you're an honest vendor your membership shows that you are. Such registries exist in other hobbies and have been successful in policing dealers and keeping them ethical.
For example, call one and ask them if a Juniper Nana can be grown indoors. Not only do the reps recommend the Juniper as an indoor tree, they'll state more than once that any tree can be grown indoors, and sure, if you want to invest thousands of dollars in lighting and don't mind cutting a hole in your roof you can grow a giant sequoia indoors (for a while). Technically, it's the truth, but only technically. They don't point the novice or the gift buyer to one of the few recommended indoor trees at a lower price point.
This and other "screw the truth and full steam ahead lets make the sale" approaches can be found among various dealers, so can the bait and switch and other scams.
(The 25 year old tree that's 15-18 years old at best is a frequent one).
I buy a lot of trees. I have friends that buy a lot of trees. We've seen it, heard it, experienced it...
"You live in Michigan? It's December? Sure, just sit that Trident Maple in a window and water it once in a while. It'll be fine. I've got a customer in Buffalo NY who's been growing one on his kitchen counter since 1997".
I understand business. I've owned one for decades, and I'm all for making a sale. But more of the wrong trees are sold to novice's every day in the bonsai biz, and there's no one to protect the buyer – and online sources can be misleading and downright contradictory in offering advice.
That said, I know some honest nurseries, sellers, and dealers I'd recommend to anyone.