Surely if they’ve not paid for it, it’s not theirs to sell?
Under English law title (ownership) and risk passes on delivery by default.
You can contract out of that position (it’s generally referred to as retention or title or a “ROT” or “Remalpa” clause).
To get a return of ROT goods you’ve got some hurdles to pass.
1) you’ve got to prove that the buyer accepted your terms
2) you’ve got to identify your goods and with a simple clause (goods under this invoice are ours until paid) you’ve not just got to be able to say they came from me but they came from me under this unpaid invoice. Even if you have an all monies clause (all goods are ours until all our invoices are paid) if the supplier was ever owed zero you need to show the goods were delivered after that date.
3) they’ve got to be worth your while collecting
4) you need to not go too crazy on the terms and accidentally create security that is void due to its lack of registration at Companies House.
There’s mention of ROT claims in the administrators proposals.
Basically if they are the Company’s (i.e. ROT does not validly apply) then you can sell them.