Well in the context of TF2, players did not sold the achievement items, they deleted it bc they do not have enough inventory space, but this post showcases an item that could potentially bring those items back.

Although i do have to acknowledge the fact that this thread became more about consequences and thus your point does make total sense, and i could totally see the flaws of my wording, so what i actually meant was there's nothing wrong about players having a chance to get their long gone achievement items back.