Murphy's Laws of Civilization, Trading Edition:

[Smokeybear]

1) If another civ is quite friendly and holds zero diplomatic ill-will towards you, they will never have enough gold to buy your goods, and will be perpetually broke.

2) If another civ is quite friendly and actually has the gold to buy your goods, they won't need anything you have to offer.

3) If another civ has endless gold flowing out of their butt like a yellow fountain, they will always decide to hate you for no good reason whatsoever, will offer you the absolute bare minimum possible for your goods, if that- and will keep that piss-poor attitude for the entire remainder of the game no matter what you do.

4) If by some fluke another civ is a perfect and lucrative trading partner, it will be quickly and brutally conquered by the civ from #3.

5) No matter how much you get extorted by other civs for a research agreement, you will never, ever see them offer YOU gold or goods to accept an RA with them, no matter how far ahead of you they are in the tech race.

This is by no means a complete work- feel free to add on as you like.

 

[comatosedragon]

6) If you do manage to sell a luxury to a friendly civ for (close to) full price, they will ask you to give the money back on the following turn. (Things aren't going so well over here)

 

[WChrisMullen]

If you completely destroy a rival civ in a war, at the peace agreement he will still want all of your luxuries and gold.

 

[compwiz1202]

If you completely destroy a rival civ in a war, at the peace agreement he will still want all of your luxuries and gold.

Grrrrr this one always angers me, especially when I'm trying to play peacefully.

 

[TheMarshmallowBear]

7) If you want take over cities peacefully, conquer their capital.

 

[Kalmah]

8) Civs value their lone luxury resource higher than achieving victory

 

[dopingman]

9) Your worst enemy will still trade that luxury he doesn't need, albeit for a hefty discount.

10) Everyone needs luxuries. Even when they have 60 happiness, MOAR LUXES PLZ!

11) Iron is still useful after researching modern armor and stealth bombers, you can trade at full price.

 

[ense7en]

9)

11) Iron is still useful after researching modern armor and stealth bombers, you can trade at full price.

Only if you are trading to a civ still stuck way behind in an older era...they don't want iron once they hit Industrial usually, if not sooner.

 

[Smokeybear]

Only if you are trading to a civ still stuck way behind in an older era...they don't want iron once they hit Industrial usually, if not sooner.

Yes, just like real-life civs, that have no use for iron/steel after the industrial era...