I have a number of items in my Steam inventory that are never going to get used; for example, some AirMech 2 "gifts" that are marked as NO LONGER VALID. Is there some way to get rid of them so I don't have them cluttering up my inventory? Obviously I could do a one-sided trade with a friend, but then they'd have to deal with it instead, and I don't have any alt accounts.
Yes you can for certain items - for example, when I look in my Steam inventory at my many copies of Dota 2, I can select one and a "Permanently delete gift..." button appears as per below;
This option doesn't appear for all items, for example if I select any of the new trading cards they do not have a delete option and the option is not present for the copy of XCOM I have from the Tomb Raider pre-order - in these instances you can gift them to your friends or, if they're marketable, sell them on the Steam market.
In the instance of your "No longer valid" gifts, I would suggest contacting Steam support if the "Permanently delete gift..." option is not available for you.
I forget where I heard about it and I haven't used it myself, but TrashBot claims to be happy to take those one-sided trades, so you can use it to dispose of undeletable items. For as long as it's still running, anyway.
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According to the group's overview:
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