I have been on both sides. One where I ordered rays and did not receive what I ordered (ray was wrong sex - this was an experienced breeder / exporter and knew what he was doing). The vendor promised to make it right and offered me a "deal" on an additional ray as an apology and then screwed me around on that ray too (didn't send what I paid for and sent a lower quality ray). I see the point of not leaving negative feedback in hopes / trust that the vendor will come good on his dealings. Afterwards I found out I was not the only one that had poor dealings with that vendor and could have saved a lot of time, money, and grief had I known. I tell others who ask me about my experience with that vendor.
On the other side, I joined a different forum and after a while posted some fish for sale. Another member posted a reply that I was a scam artist and sent him a dead arowana that was not even the one he ordered and derailed my thread ( I've never sold an arowana in my life). He had mixed me up with another member that had a similar name as mine who had been banned. Instead of Sending me a PM first to ask who I was, he discredited me " in the sake of warning others". After he realized his mistake, he posted a reply apology.
There is valid points on both sides. Most member on this site know other members and you can PM them to ask for recommendations /advice if you suspect something. This doesn't usually help new members who are just getting in the hobby and don't have connections. They are usually the people getting burned.
I don't see a problem with someone asking advice or feedback regarding a vendor or member on this forum when it comes to wanting to purchase a ray. The reputable members will clearly be evident and the unreliable ones will be flushed out. One off incidents sometimes happen, but when you see repeated negative comments that should be a warning sign.