Let us say the AI is willing to trade you 10 gpt for fish or whatever. Okay. Fine. Then you carry out your 2-step exploit and suddenly you get for example 100 gpt.
@Pangaea: This isn't quite right....unless you're suggesting that you rinse and repeat across multiple AIs.
Just to clarify, what's actually happening here is something along the following lines:
Step 1: You (ie. the human player) trade away any resource of which you have only one. Just for illustration, let's assume you have just one corn, which you therefore trade away.
More importantly, you also agree to give a lot of GPT away to the AI. Let's assume for simplicity that you can manage to trade away 10 GPT, as per @Pangaea's example.
In exchange, you request a resource from the AI. That resource you request in exchange can be anything and is irrelevant to what's going on. Let's say you receive bananas in this trade, just for illustration.
So, to summarise: this step ends with a trade in which you sell your lone corn and 10 GPT to an AI in exchange for some bananas.
Step 2: @TMIT then suggests that you sell as many other resources as you need to (bearing in mind that you sold your lone source of corn in step 1), to the AI, in order to receive 10 GPT from it. This is course is the very same 10 GPT you just traded away to the AI in step 1. Of course, if the AI is willing to trade you more than the 10 GPT you just gave it, you'd trade for that too.
As an aside BTW, it's worth noting that if people are getting confused at this point with what's going on (with all the resource trades flying everywhere), you can consider a very simple variant on @TMIT's step 2: imagine you sell only one of your other resources (let's say gold, just for illustration) to the AI, in exchange for the 10 GPT you just gave it in step 1. It's completely possible to consider this alternative BTW, because the only reason that the player will likely need to sell more than one resource to the AI, to get back the 10 GPT at this stage, is because the amount of GPT that the AI will give you in trade is capped at 10% of it's population (as @TMIT notes). This abstraction does not change the logic behind what is going on – instead, in this variant, I'm simply implicitly assuming that the AI's pop is 100 or greater at this point, so that it can offer the very 10 (=10% of 100) GPT just given to it in step 1.
Step 3: You then pillage your corn. Since it is the only corn you had (and this is why you have to trade away your only one of a resource in step 1), the trade you made in step 1 is now cancelled. Remember, that trade saw you give the AI your lone corn and 10 GPT in exchange for some bananas from the AI.
However, the AI now has a very clear problem. The trade done in step 1 has been cancelled because you pillaged your lone corn. However, the trade (or trades) done in step 2 are still active. In other words, the AI is still committed to giving you 10 GPT in exchange for whatever resources you sold in step 2. The problem for the AI is of course that it is no longer in receipt of the 10 GPT which you gave it in step 1, because that trade was cancelled when you pillaged your only corn.
As I mentioned in my previous post, the real issue here is that the AI does not now recognise that the optimal thing to do is simply cancel the trade(s) you did with it in step 2. In short, the AI does not recognise the dependency between the trades in steps 1 and 2 above: you gave it GPT in step 1 and have traded for it all back in step 2. Having seen it's GPT taken away from it once the player pillages the resource (here corn), the AI should simply realise it no longer has the GPT given it in step 1, to fund the trades in step 2. However, because the AI does not recognise this and act accordingly to cancel the trade(s) in step 2, the AI will now have to find that 10 GPT in some other way, perhaps by trading with another AI, building cottages etc.
FWIW, I did not comment on whether this is an exploit in my earlier post – and quite deliberately so – because, as @TMIT notes, many gamers already work to take advantage of AI shortcomings in so many other areas anyway. As a result, I guess you could make an argument that the gamer is just playing to leverage yet another AI shortcoming. That said, it is not a strategy I have ever pursued, nor do I feel compelled to use it. But, each to their own, I guess.
The one point I do want to make to @TMIT however is that his assertion that defining this strategy as an exploit means that tech trading is also exploitative, is a false one. As mentioned in my earlier post, I can see little, if any, causality between these two points. A simple tech trade is an exchange of resources (here techs / gold / maps etc.) judged by both participants to be in their best interests – and whilst it is true that some may grumble about the AI logic used to value an individual trade, that is not at all relevant to what is going on here. Instead, what is happening in this strategy is that the AI is failing to recognise a dependency between the trades in steps 1 and 2 above – something that is explicitly recognised by the human – and the AI isn't cancelling the trade(s) in step 2 when the human triggers the cancellation of the trade in step 1. IMHO, that is a very different issue...and to realise this, you only have to realise that the very same issue would exist even if all the individual trades in steps 1 and 2 above were completely fairly valued.