I have to agree with Lily_Lancer on this one, if you are able to take all the AI's gold for nothing, that sounds very broken to me.
Well, pls look at my other thread about trading diplo favors. Same for resources, the problem is not only that you can sell them for 1 GPT, is that you can buy them for less than 1 GPT, so you're basically earning money while paying nothing. And the process can be cycled, resulting in you getting everything as soon as you meet an AI.
I don't use the bug you posted where you buy and then resell. honestly, I ain't got time for this ****. in my game I simply tested selling strategics/diplo/luxes for lump sump vs selling one for 1 GPT. I did not test the specific bug you are referring to, and frankly, even now that I know about it I won't be using it.
How comes using that don't give you a better finish time?
there are many reasons why lump sump can be superior or why trading 1 X for 1 GPT can be worse.
1. crippling the AI economy also means crippling their development, so the AIs economy in the future is damaged, hence "abusing" this mechanic means you will get less possible trade gold
(!) in the future
2. in fact with selling 1 strategic/diplo for 1 GPT you will not just hinder their development, but actually have them go in the negative. often times they do not get out of the negative for the rest of the game. it also means their units dissolve, they often have trouble with barbs as a result, crippling them even further. it is a negative snowball.
3. lump sump is better than GPT, because lump sump means snowballing sooner. 30 gold is always preferable to 1 GPT, but sometimes even 15 or 20 gold might be preferable to 1 GPT. I want the gold now, not in 25 turns.
4. trading for GPT instead of lump gold can be very bad if an AI declares on you, or if you want to declare on an AI. it can also be strictly OP if you sell your own GPT, but that is yet another mechanic which we are not discussing here.
5. trading strategics for GPT can make your game crash and corrupt your savefiles (not really efficiency related, but a general concern)
so yeah, there are a lot of reasons why trading 1 X for 1 GPT is not always ideal, especially in the long run. in Civ 6, as weird as that sounds, you want the AI to be strong, just like you did in Civ 5 for tech steals, you need a strong AI in Civ 6 for late game deals, making Big Ben money, making Spaceport money, et cetera.
in the end, my experiment showed me that trading 1 strategic for 1 GPT will definitely create a stronger economy, but you will have a worse early game and a slightly worse lategame. all of your rushbuys will come online a few turns later, which is crucial. buying yourself a slinger/warrior/scout on t20 or on t24 makes a huge
difference. with the 1 for 1 method you will definitely have a stronger income, that is beyond doubt. you will also have fewer income spikes, which I think is more important. really, really efficient players use purchasing discounts for everything in Civ 6. that means you buy tiles almost exclusively with the policy card slotted in, you save up massive money for democracy rushbuys and you time your big ben. I understand most people do not want to do this, but this strategy works much better with lump sum in my opinion. thus, lump sum trading might actually be optimal play under certain conditions.