The old point of view was that the best course of action was to grind them down into stubs. but the more recent argument is that the gold will quickly wear away with use, and at some point it will loose so much that it effectively cannot be retipped. so it becomes an ethical argument.
i think if its between melting a nib down and grinding it to a stub i think grinding it would be the way to go. if its a rare or uncommon nib that still is not worth retipping i would say to sell it or hold onto it until it is worth doing, but if its a really common nib with a really low value, like an eversharp skyline nib. the one exception i would make to this would be a low value nib that is flexible. vintage flexible nibs are prone to damage, so they might be worth hanging on to.
There was one site that would buy damaged nibs for over melt. I thought it was fivestarpens, but i did not see it on there "what we buy" page. if you can figure out what site that was that might be the most ethical way to sell them.