One of RQ's Second Age books mentions a place where elves trade with men just to keep an eye on them. They don't give a XXX about what they get, it goes piling up in some remote warehouse - but it keeps a communications channel open with crazy humans.
In another, elves ask for friperies, jewels etc. but a godlearner revealer team think they use these items to prepare a Quest against humans, and want to stop the deals, to the great anger of local tradesmen, who in turn sabotage the GL's investigations.
A few things that could interest elves are grains / plants that don't grow in their own forest ; petrified branches ; out-of-season delicious fruits or flowers ; any fertility magics, if only to examine it ; and unknown parts of the Song of Aldrya (which is all of the above really). Copper weapons might also interest them, if made purely (i.e. by sun folks).
But the more interesting question would be : what do you/your PCs want to get from elves ?
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