Looking at the requirements, item is definitely a relevant element
for my system, so it is appropriate to model item as
a class. Sale, on the other hand, is more of
a characteristic of an item, an attribute of an item, rather
than a class in itself. So, we're not going to mark
this one. Shopping cart sounds, as well, as an important element
for my system. Time can be an important system in some
contexts, but in this case we're measuring time just because more
than one item at a time can be added to the shopping cart.
So, time really doesn't have any reason for being modeled as a class.
And finally, user also seems to also have an important role to play
in the system, and therefore we will model user as a class as well.