> What would be the purpose of the clause if authors/inventors couldn't sell their work?
Not that they can't sell their work, but that they can transfer their exclusive rights in such a manner that they themselves can no longer exercise those rights.
And laws which allow this are obviously, by definition of the words at the time, not "securing" those rights to the authors and inventors. You can't secure something to someone by allowing them to (both post-hoc and especially pre-hoc) permanently transfer those rights.
Especially the pre-hoc transmission of rights as seen in various contracts (work for hire, etcetera) completely flouts the "securing" basis of the constitutional IP clause in that under no circumstances whatsoever did the rights ever belong to the author/inventor. The Constitution did not grant congress the authority to make laws allowing intellectual property protections to a pre-hoc transfer, but only possibly to transfer in which the author/inventor first had exclusive rights. Therefore any contracts which automatically assign any rights to any creations to another party the moment those creations are created cannot be covered by intellectual property protection laws created pursuant to Article 1, section 8, clause 8 powers, but must have some other constitutional basis (possibly under the commerce clause?).
I guess you could get around this argument fairly easily by saying that the "limited time" is X number of years or until sold. But then I still don't understand under what authority the purchasing party gets these IP rights, as they are not the author or inventor, and thus not entitled to any securing of rights under this clause. If they do have rights it must be under either common law which pre-existed 1789, or under another section and clause of the Constitution, since this is not a 9th or 10th amendment issue. Congress can't just make law willy-nilly however it wants to, but must do so under one of its enumerated powers.
I guess another way around this, and possibly the way it is done in IP law (you would know better than I), is that these rights aren't literally sold to another, but instead a permanent licensing contract is signed in which the author/inventor grants use of their exclusive rights to the licensee to do with however they wish. In this way the "exclusive rights" would still be secured to the author/inventor, though exercised by the other party. This seems like sophistry, but much of the law is.