Anytime a gold weapon isn't max damage for its class, you should sell it as unIDed (such as a 8-16 damage sword, for instance). Also, gold armor (depending on how lucky or not lucky you feel about getting a good rune).
For skins, it is a good idea to check this thread to see what skins are rare and you should keep those. Usually you want rare skins with low reqs (9 preferably).
Updating answer to address "do the requirement skill for holding the weapon matters as well?"
Yes, but only for extremely rare skins like the Bone Dragon Staff. In those cases, even ones with useless skill reqs such as Smiting are worth less, than the more useful skills such as 9 Prot or 9 Heal. Even still, because the skin is really rare, it's still worth a bunch of ectos. General rule of thumb, if it's on that list in the link I provided above, it's worth something. If not, then if it's something rare like Req 8 then it's probably worth a decent amount as well. Other than that, you can choose to either ID for the title or hope for a good mod to sell (like +30 hp), or just sell as a junk id.
Oh, and one last thing, Canthan-only weapon skins tend to be worth more since they can't be modded with inscriptions. So if you find a 15^50 weapon with reasonable req (like 9 or 10), it's worth getting some opinions on whether or not the skin is desirable. The value has since dropped a bit due to the fact that the HoH chest can drop them in the inscribed form but they are still in good demand imo.