I think you guys are missing the point of diplomacy - it’s not to make friends but instead to obtain gold from other empires. While you can’t sell technologies any longer, the AI will often pay you quite a bit for resources and cities.
Early game, I sell any resources I can get to whoever will pay the most as long as the sale won’t drop me into negative happiness. A golden age early is a waste as you don’t have enough population to work enough tiles to make it worthwhile, and each age just bumps the counter to the next subsequent golden age higher.
I’ve found it’s a bit tricky to sell resources and get what you want (gold) since the “What will you give me for this?” button will often times cause the AI to offer a variety of things. Here’s what you do:
How To Cash In on Resources:
Put a single luxury resource or start with about 5 of the strategic resource you want to trade and hit the “What would you give me…” button.
-If the offer is purely in gold then it’s the best you’re going to get. Accept or decline depending on if the gold offer would be useful.
-If they throw in Open Borders on their side and you don’t want it, manually remove it and then bump the Gold Per Turn up by 1 or 2 then retry the “Would this work?”. Keep fiddling until they re-agree to a gold amount sans Open Borders. I’ve found Open Borders is usually “worth” 1 or 2 GPT to the AI.
-Likewise, if they want Open Borders from you and you don’t want to give it, remove it and decrease the GPT offer until they stop asking for it.
-If they offer any other resources and you don’t need them, remove them and fiddle with the GPT until they agree again.
-If it’s a strategic resource, remember that you can adjust the amount you’re offering up or down as well to make a deal work.
-Also with strategic resources, sometimes the AI will refuse a trade altogether simply because they don’t think they have enough gold to trade you for what you’re offering. If they flat out refuse to consider a trade for 5 Horses, try 1 Horse and see if they’ll bite, then adjust upwards from there.
-If the “Would this deal work?” button disappears, just remove and readd anything from the trade and you’ll get it back.
-Remember that upfront gold is better than GPT, because GPT will end in the event of war or if the Civ you’re trading with is conquered.
Assuming the AI has gold to cover it, you can reduce any trade into a purely gold trade, and the AI values resources insanely high (as you may notice if you try and trade to get one FROM the AI). Remember that you can get gold from an AI for resources, then use that gold on a City-State to get resources back to replace the lost happiness plus get the City-State bonuses.
Another “diplomacy” technique is that after a war, puppet and then immediately resell the captured or surrendered cities to other empires - absolving yourself of all unhappiness and maintenance and making bank instead of razing - they will gladly take them and pay you handsomely (you can recapture them later if you need to).