You can sell your key but making copies of someone else's map (without permission) and selling it is going to violate copyright laws.

Seems pretty shady to scan in someone else's map, remove their title and author's name, add a key and sell it as your own.  I know I wouldn't buy that.

Mars wrote in City of Valla key questions:Seems pretty shady to scan in someone else's map, remove their title and author's name, add a key and sell it as your own.  I know I wouldn't buy that.

That would be going too far IMO.

For clarification, what I'm saying is, legally you can build a piece of work upon someone else's work, because they cannot copyright the intent of the image, only the image itself. I can draw a picture of my neighbour and he cannot sue me for copyright infringement. I can publish my own rendition of Greyhawk, or the Keep on the Borderlands, or Yggsburgh, and fill in the blanks where I found the information lacking in my campaign, and it is all perfectly legitimate and infringes no-one's IP so long as I do not use the words 'Greyhawk' or 'Yggsburgh' or the phrase 'Keep on the Borderlands' which I imagine WotC would be able to successfully protect in the RPG arena if not anywhere else.
It is fair use to publish supporting products for already existing products where there is no such product so long as you don't infringe anyone's IP. How many people have published Decks of Many Things or any number of accessories into the market in support of RPG gameplay where the originating publisher was deficient in meeting market need?

In this instance we appear to be talking about a city map which from my recollection is a fairly well proportioned and designed map. The author has not followed the map up with any supporting information to allow anyone to use the product, and Zardok has suggested a key for the map may be a good idea. As I recall this is something he has done in the past to some success working with other notable authors. Obviously, in the first instance you try to find the Author. I'd imagine he is still contactable in light of old stock resurfacing in quantity five years or so ago. So asking the question directly would be the sensible first step. Should that not be successful though, that does not kill the idea. A key where one does not exist and has never existed in the marketplace is a perfectly viable prospect. Having a map to go with that for those who cannot own an original is also a perfectly reasonable thing to offer, provided it is not a facsimile of the original which is a breach of copyright law. And of course, offering a key without the author's permission and using the title 'City of Valla' is a clear breach of copyright also.

All said though, how much money is in the sale of a keyed city map? As a digital product  you're looking at maybe $1.99 a pop, of Pay What You Want?