Good question.

If the rental ends as scheduled the card goes right back on the market automatically and is immediately rentable by the next person no matter when or if the person before them used it. The only time this matters is when the card owner cancels the rental and would need to manually relist the card. Its the relisting where we find the restriction.

If you rented a card from me for one day on saturday at 7pm for 10 dec and on sunday at 5pm I noticed this card was going for 35 dec, I might want to cancel that listing. If I did that, you would still get the card til sunday 7pm but once it ended (as scheduled), instead of the card automatically relisting at 10 DEC for the next person, it would now come off the market and go back into my deck where I would hope to be able to relist it at 35 dec and take advantage of the spike. But, if you used the card (or I used the card when it came back to me) I would have to wait 24 hours from whenever its last use was before I'm allowed to relist it.

So if at 5pm I see you hadn't used it and I cancel expecting to be able to relist but then a few minutes before it expires you do use it, I'm sitting on a worthless card for the next 24 hours. I'm honestly not sure if I could use the card myself at that point but if I did it would just be prolonging the window until I could relist it so more than likely the inventory will just sit there.