Sisiutil
All Leader Challenger
I don't necessarily agree. Don't trade a wonder resource that helps build a wonder you're going after, no. But if you're not/won't be pursuing a wonder, and a resource or GPT you could get in trade for its accelerator will do you some good, go for it. If you trade it to a nearby civ, maybe they'll build the wonder for you (instead of building units) and you can go capture it. Also, later in the game, stone and marble become much less valuable once the wonders they accelerate have been built, so you might as well get something for them.Try to trade them but NEVER EVER trade a wonder resource or an offansive resource, like iron, copper, uranium, stone and marble... but work the resource tile if they are in a city's BFC, since it gives massive hammer or commerce bonuses...
I often like trading copper once I have at least one source of iron. The AI values it highly, so you can get a lot for it. The only time you have to worry about it in relation to a wonder is for the Colossus and the Statue of Liberty.
I've also sometimes traded horses or iron to a distant civ I have no plans to fight with--especially if they will, in turn, cause problems for a nearby mutual rival. But I never trade horses to a nearby civ, or even one on the same continent if I can avoid it. The AI just loves mounted units way too much, and while they're not devastating, they are annoying.
Finally, there can be definite advantages to trading/gifting strategic resources to your vassals. The stronger they are, the better allies they'll be in wars against your mutual enemies.