Can I sell mysterious constructions without upsetting their creators?

If one of my dwarves is taken by a fey/strange/macabre/fell mood and builds some wondrous trinket of no utility, can I flog it to a passing trader in exchange for some more booze? Or will the artifact's loss lead to bad thoughts and potential !FuN! ?

You cannot sell artifacts at all. They can be atomsmashed, stolen, dropped into the center of the Earth or otherwise removed from the map/existence, however. The loss of an artifact will lead to unhappy thoughts, but I believe the same dwarf having made many masterworks will lessen the impact (IIRC unhappy thought strength on loss of artifact/masterwork is 1000/total number of artifacts/masterworks by that dwarf).

If you put an artifact in a heavy container (lead/gold/platinum bins are good for this), they will remain in the same place after the fortress is abandoned, which means your adventurers can sell them for a good bit of pocket change.