Seems to me that the negotiated and agreed price between a buyer and a seller isn't really anyone else's business. Providing that info just invites negativity unfortunately. Someone is often going to think the buyer paid too much and the seller took advantage of them (drawing criticism to the sellet), or that the buyer got a steal and the seller didn't know what they were doing (drawing jealousy for the buyer).
If you look at pin average prices on Pinsider, it might be a reasonable start,but if you look at sale details, prices vary wildly regardless......often huge differences in sale price for a machine that, at glance, looks just like another sale for a very different amount. There are too many factors (local availability, how bad someone wants get or get rid of a machine, etc) for transparent pricing to mean a whole lot. The right price for a machine is the price both parties ultimately are happy with.