It burns me up when an online vendor's "cart" isn't really a cart. Meaning, if I go to the brick and mortar store, take an item off the shelf, and put it in my cart, that's where it stays until I get to the check-out counter. Nobody can grab it during that time (well, not without a fight anyway!). Now, if I leave the store with items in my cart (left behind) without ever checking out - well, those items go back to store inventory and can be bought by others.
There must be something to website design that precludes making online carts work the same, but I don't know why it couldn't be, and maybe one hour without any site interface activity could equal "leaving the store without checking out" - I've no expertise in that field but that's sure how I would try to design it if I was setting up a vendor's website.
Rex