Indeed, "salvage" seems the correct option for this, which was obviously common, from reshaping metal tools (an advantage over stone ones) to reuse stone from many great wonders for new structures. Ships would be sunk to make the base for port extensions, and some think Archaic Greeks would turn over and raise their ships to build wooden temples, while the Rhodians dismantled and sold siege machines to finance the Colossus, since you mentioned them. I would implement this in reverse to their production: salvageable units should have a salvage button when garrisoned on the building that produced them (and take maybe the same as production time), while builders should be able to salvage structures (this working in reverse of building them). The return should be some fraction of the materials used in the first place (maybe except food). An independent thing could be the ability to sell certain units to other players, for metal.

I would call this feature not sell but salvage for resources.

If we want selling not to be too simple and rewarding, let's imagine and make the process slower — say, twice the build time — as if you're "searching for buyers". And of course it means that each ship should be manually garrisoned at a shipyard (siege at a workshop) and player needs to press specific button.  Anyway it would be cool and intriguing in my opinion.