The AI certainly does trade maps with themselves, and that is one caution about trading your map. If you are going to trade, try to trade to everyone the same turn, to get the most gold. The first turn you trade it, you will get the most value. I don't know the exact formula, but basically the AI values it based on the number of tiles you have revealed that they have not revealed/seen yet. So the more you have explored, the more valuable it is, sometimes even being worth a tech or two if you have explored islands or regions that they have not seen. Also when you get someone else's map, that is added to yours, and if the AI have not made contact with them yet, those are all new tiles and get added to the value. For example, in a current game I made contact with another continent, and got maps and contacts with 5 AI civs. I then was able to sell my map, which included all their maps, for several hundred gold to the civs on my home continent. But if I only sold it to one civ, or sold them contact with another civ, then the next turn my map would be worthless as they would have already traded that information around. That's why if you have a large map, it may be a good idea to wait on trading it until several AI have money or techs to offer, instead of selling it to the one AI with cash, and watch him trade it around until it is worthless.