I bough the php & mysql for dummy's book and worked from there - however, 5 years on and although I have written e-commerce systems I can tell you they a not first-year projects at all.
they are far bigger than just adding things to carts. you must consider shipping and returns (possible stock control).
also ... A sale requires the prices of each item to be stored as a fixed price rather than a reference to the items current price - a price is fixed a point of adding to cart not a week later when the prices are changed and they want a refund at the old price - or you want to calculate the sale for any given period etc.
all product prices must be stored ex tax along with the applicable tax rate stored separately -and individually for each item.
shipping tax is calculated independently from product price so you must store these separately.
...and they require knowledge of the legalities of 'distance sales', local and international taxation.
oh.. can a whole heap of canned worms are being let loose here dude...
i'd advise you learn magento or zen-cart / oscommerce / cre loaded etc.. then learn php / mysql then learn to develop mods to these stores then write your own ecomm.
hope i scared you enough