In the past I have PM'ed a seller to let them know a price may be high. Its usually a new person, or someone who has been out for awhile. For example, guy had a SP-1, stock and was asking $150ish. After several days of him bumping his own thread, I POLITELY PM'd him and let him know the reason it was not moving was the price was a little high, as you can get them on ANS for less, and how much I have seen them go for, as I have a couple myself. I also told him he can ask whatever he wants for it, and most times sellers are responsive to this.
BUT I only comment on things I know and are familiar with.And only im PMs, NOT in the thread!!!!