I don't think posting your experience good or bad with a seller is prohibited...if just the facts are presented and no name calling or degrading comments are used. Personally I don't think relating a good experience is actually promoting, as long as it is an accurate representation of the transaction. Throwing in over blown praise or negative comments I am sure would not be appropriate. I am sure even facts could be over done with a lot of repetitions, no need for each transaction to be presented. But general comments like " I have always had good service and great plants delivered in a timely manner" I personally think are fine. Of course what actually is allowed depends on Dave.
Saying that you ordered plants back in September and still have not received them in November and have not been able to make contact with the seller I think is a service to the rest of us, just don't call them a scammer or a cheat...etc. Let the others decided if they want to buy or not based on actual facts.