they never give you a negative for negoatiating. there can be a bonus for giving them the better end of the bargain, but it's capped. the max for "fair and forthright" (tech) trades is +4. you can also get a +1 (perhaps more, +1 is all i've ever gotten) for "you have shared your technology with us". that happens when you give or trade them a monopoly tech, if they're in a generous mood enough to acknowledge that. there are other modifiers for resource trades.
the general rule is that they appreciate deals that are weighted more in their favor, and give you more credit for them. but some leaders just don't put a big priority on trading, so they're hard to get up to +4 and then they fall from +4 to +3 and so on fairly quickly. you kind of get to know who's worth bothering with and who's generally a lost cause. in my current game, i gave washington 2 freebie techs to try to increase global tech rate, and 2 super-tilted tech deals in his favor since i needed money to upgrade troops after victoria turned evil, and he's still not at +1 fair/forthright
. after my very first tech trade with mansa, he was pleased as punch and now spares gold for a friend every time i call to ask, even tho we're not siblings of the faith. mind you i'm no heathen, i just have no state religion. but mansa really considers tech trading a big priority in life and everybody who participates in it is okay with him i guess.
you can get to +4 even while negotiating and squeezing every little thing you can out of tech trades, as long as it's an AI who appreciates tech trades and you have enough of them to do. what i love is when i ask "what will you give me for X?" and they put tech Y in the trade window. then i take X out, put X back in, and click "what would make this trade work?" instead of "what will give me". often they add in all their gold. ok honey, if it works better for you to pay me money, i'm willing to take it
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often i'll meet someone late in the game who is of course annoyed with me since i traded with his worst enemy before i knew he existed. if he's behind a few techs, i'll sell him one that everybody has, for some low amount of gold. it doesn't have to be a free present, just a really great deal for them. theology for 200g, whatever. that can often get you an instant +4 "fair and forthright" and move them from annoyed to cautious, and hopefully willing to trade when/if they do have techs you don't.
