[Someone else may have posted this but I'm new to the forums and couldn't find a mention. Apologies if this post is redundant.] To replicate the exploit, offer 1 coal to an AI and ask for 10 gold/turn in return. The AI will refuse. Ask to make the trade more equitable. The AI will offer 5 gold/turn for the 1 piece of coal. You can repeat this until the AI runs out of funds. You can then often buy the coal back for far cheaper than you traded it for and repeat the process. When testing this, I had at one point been gaining upwards of 700 gold/turn. It doesn't seem like there is a limit to the exploit. I've heard that it can be done with other resources such as oil and possibly uranium, though I don't believe trading iron/horses is exploitable in this manner. I've seen this exploit discussed on Youtube and other forums, so I'm hopeful it'll be fixed in the next patch. For now I'll just have to avoid trading some of my strategic resources.