Hard to say what it is worth. E-Bay is a barometer but not neccessarily a proven source for a pieces value. Rack systems like this are almost never sold as a single piece so that creates one problem and then the second problem is whether enough of them have been sold on E-Bay to firmly establish the market.

The Kenwood KM-series amps (I think you have at least one) were part of a rack system which was apparently fairly popular given the number for sale on E-Bay. When you look at the completed listings for them they seem to sell in a consistent range. If you try to find info on this Marantz amp you notice there is not anything out there except the one you mentioned.

I think this is a case of a seller trying to take advantage of a name rather than relying on the quality/market of a particular piece. You see something similar with Pioneer gear of the same era. I am trying not to rain on your parade about this piece but I don't think you have something here that is well sought after by Marantz collectors but if it sounds good to you then that should be what counts.