is the rental of an item limited to just one day ?
Here's a sample I did a while back; not totally tailored to your description*, but it may still give you some inspiration.
* You could of course change the selection portal (or the underlying relationship) to show only items that aren't yet selected, or where onHand > 0, or whatever other rules you want to apply.
Yes, it would be limited to one day.
Wow, this is amazing, thank you very much. I'll have to play with it and see if I can make the inventory refresh with each day.