The only portion I consider an exploit is the process of making deals before wars in order to rob the A.I. of gold... Otherwise I think gold and luxury trades are a pretty viable strategy since there are advantages to the A.I.
- The Sale of luxuries = More happiness in exchange for the abundance of gold A.I. usually have. As I understand it on higher difficulties they get a greater amount of happy per resource and while in most cases they may not need the happy for expansion purposes, so you're effectively exchanging gold for a quicker Golden Age rate, which will probably recoup their losses.
- GPT for lump sum = I've done this quite often, especially when I've found myself in cases where I expanded too quickly without an appropriate military and my neighbors decide to take me out. I find this fair game because often times the deal has about a 50% interest rate. If I get 100g, they ask for 5gpt or more. Which, even if the deal doesn't last the full 30 turns, it'll probably cover their loss at least.
- Open Borders ~ This is the most arguable of the ones to sell and consider an exploit, however, I have had one way open borders agreements with A.I. who have seemingly used it to position themselves for an initial assault in an inevitable war. One might argue we as players would notice this but honestly I was caught completely off guard because it wasn't a centralized force, there was a unit here, a unit there, etc. on different ends of my empire and it cost me 2 workers and a settler en route to a new city site that I was backfilling within "my" borders. It honestly looked as if there were just a couple of random units exploring the map.
but I don't shortchange the deals I make with the A.I. players and have found it to be a very viable way to fund the early game and I quite enjoy the fact that I can take out loans. In one game I almost got wiped out by my two neighbors cause I completely lost track of how much time was passing in the game in relation to how much military I had. I had already sold my luxuries away for cash that was used on other things, but my GPT was pretty sizable for the early game (had a lot of gold resources across my empire), So a few quick deals that gave away literally all my GPT in order to buy my military brought me back into the game.
That same nation that took my GPT eventually found itself leading the scoreboard and being my biggest threat, so I don't feel as if I cheated the system.
Personally, if there's any change to the system, I'd enjoy seeing trade deals last regardless of war status. Exception being Research Agreements.