Deity AIs often have 1000-2000, sometimes have several thousand, rarely have more than 10k -- your last rival in the late game might, but probably no more than one. Or sometimes they'll be broke, because I've already extracted all of their money, or they've signed it over to another AI in a peace deal. If I'm playing for Domination on a large map, I expect to declare ~8 wars over the course of ~120 turns, so approximately once per 15 turns, starting around turn 90 (if war breaks out earlier, it probably came to me unbidden). So that's the scale of the numbers. Luxuries are pretty minor on this scale (I'm likely already selling most possible luxuries), but except for the first war, gpt could often be used to extract all of their cash-on-hand before I declare on each subsequent victim.
If you're going to do this, you can multiply your proceeds as follows: Trade all your gpt to civ A for cash. Trade the cash (plus any cash you started with) to civ B for gpt. Trade that gpt to A for cash, and back to B for gpt. Repeat until either A is out of cash, or the haircut that you take on each transaction has whittled you down to nothing. Declare on A, and enjoy a large income stream from B. The theoretical limit is 1/(1-22.5/31.5) = 3.5 multiplication of your gpt stream, and each 100 cash that you start with gets converted to 11.1 gpt for 30 turns. (In practice you'll do slightly worse due to rounding).
So, roughly tripling my income twice (a thirty turn boost repeated at 15 turn intervals) means a nine-fold increase, subject to the limits of my victims' cash positions as discussed in the first paragraph. That's ideal conditions, a more realistic value might be three- or four-fold. Even with "legitimate wars", with the intention of decapitating my victims towards Domination, it feels exploitative to me.
My personal moral code -- I don't generally submit HoF games, 'cuz I don't actually play them out once victory is assured -- runs something like this:
Don't self-pillage, or deliberately let my resources be pillaged. If they do get pillaged, it's OK to resell.
Anything's fair game in anticipation of the AI declaring on me. Heck, sometimes I misread his intentions and end up taking out bad loans. Plus if he really is planning to attack me, he likely won't pay much for my stuff.
If I'm planning/anticipating declaring on an AI:
I will sell resources only if I would have sold them to someone anyway -- I normally keep as many of my resources as I can on the market -- but won't go into negative happiness or strategic resource deficit if that isn't something I would have done otherwise. I don't trade gpt for his cash -- as I said above, it "feels" exploitative to me. (The better fix would be to have your reputation with other leaders take a hit if you renege on your promised payments, but until that's implemented....). If I happened to have a gpt-for-cash deal from some previous time before I anticipated war, though, I don't need to hold off my DoW just because of it. The gray area for me is selling cities. I'm usually OK with it, as there's a cost associated with recapturing those cities -- I'm deferring the march on his capital by a few turns, giving him time to marshal defenses. But sometimes it does feel like somewhat exploitative, or at least edging into gray.
I don't declare "phony wars". Actually I feel like the diplo costs are high enough, and the lost opportunity for a Research Agreement, and AIs won't make peace easily unless you're actually punishing them, that I doubt phony wars are particularly profitable in my games.
Anyway, that's three cents (damn inflation) from me. I have no idea how to implement these into enforceable rules
If it could appear like something we would consider an exploit under the rules, we are probably going see it that way upon review.
You didn't say how much GPT that was or how much the AI generally have available in mid to late game on Deity. 
