FWIW & IMHO:
E-bay by itself is a not a very good indication of values of our cars, and probably not of any other cars either. However , prices reflected in a combination of E-bay, Auto-trader, Collector car trader on line, Cars on line, Kirban's, Hemmings Motor News, Turbo trader cars for sale, & Deals on Wheels do reflect that these cars are going up gradually in value. Not drastically escualting, but steadily climbing. People can ask what they want to for these cars, but probably not without reproach when they are for sale on any media publication. If somebody buys it, fine, and if not, oh well. And as much as I hate to say this, as I know it may not be received well by most of you, this could also be somewhat of a bear market for cars such as our. People could be buying only because they see someone else buy, just like as in the stock market, and that could be the primary driver for the increase in sales/prices. Basically, as stated above, "many are called, but few are chosen". I have seen the GNX/GN/T-Type/Turbo-T/TTA market go up and down over the last 10 years or so, but in general, values of our cars do seem to be on the rise in general. No flaming intended here, just my viewpoints.
Notice that Kirban's has only 1 turbo car for sale now. And some of the less than ethical sites enter a car several times just to make it look like it has a lot of cars on its site for sale. For instance, Cars on Line never deletes a car, even after it sells. These sites can't get hits if they don't have cars for sale on them, so they delete cars as little as possible.