Is it possible that you are alternating shopping between ebay.com and ebay.ca? If you shop on one, the items you put into your cart will not present themselves if you log into the other. (At least in my experience.) Try logging into one and seeing if they are there. Then try logging into the other.
Don't forget to 'save for later'.
Also, many times a seller has only one of a particular item so if you wait too long, it will just be sold and vanish from your cart for that reason.
