In general if 2 people complete a trade/sale and it's done without issues, it's pretty petty to not leave a feedback because you later don't agree with the price you, yourself initially agreed to and then you come into this thread to discredit the seller. I, myself have bought from this seller and had no problems with them at all, I asked to see the cash, got my wish, paid upfront, like requested, instantly got my money.
That you suddenly want to have a hizzy fit about something entirely unrelated to your transaction and legitimacy of the seller is something you can keep to yourself. It's not constructive, it's not factual and it only sounds like sour grapes.
There are enough people that are not honest that you can actually, legitimately leave negative or constructive feedback (warning players from dealing with) to and you should leave the ones that are honest alone.
Also it's like you say yourself, it's business, business was done, you made it personal for whatever reason by doing this.