Do you really like an item but you're undecided yet or still want to look around a little bit more? Then all you have to do is to put it on your Watchlist.
On every item page you will find the 'Add to watchlist' button underneath the green 'Make offer' button. One click on it will add the item to the Watchlist, a second one will remove it again.
You can manage all your watched items by clicking on your username at the top right and choosing 'Watchlist' from the menu.