Back in those days, TSR protected their D&D trademark ferociously and would clamp down on anyone who tried to sell something associated with the game. However, they did license use of the game to a company called Judges Guild, who promptly brought out material of a higher quality than TSR was producing.
My friends and I dearly wanted a copy of their main product, City State of the Invincible Overlord. This wasn't so much so we could play it as to see what was involved in it. If we wanted a create our own cities (which we did), what would we need to include? It was therefore more like source material than playing aid.
When I got older and had some actual money, I came across a copy of the revised version of the City State of the Invincible Overlord booklet at GenCon, which I promptly bought at a knock-down price.
Copies of the pre-revision version of City State of the Invincible Overlord sell on eBay for hundreds of pounds. Copies of the revised version I have sell for over a hundred.
Unfortunately, I chose to annotate my copy so I could locate where the streets it referred to were on the maps that came with it. I could probably still sell it, but only at a knock-down price similar to that I paid for it.
