Is it possible to buy half of a full bitcoin? I've noticed that certain products online do not require a full bitcoin to purchase. My question is do you need to start with one full bitcoin in order to make a purchase of such items?
No, you can transfer fractions of bitcoins easily. From the Myths section of the Wiki:

One Bitcoin is divisible down to eight decimal places. There are really 2,099,999,997,690,000 (just over 2 quadrillion) maximum possible atomic units in the bitcoin system.
The value of &quot;1 BTC&quot; represents 100,000,000 of these. In other words, each bitcoin is divisible by up to 108.
As the value of the unit of 1 BTC grew too large to be useful for day to day transactions, people started dealing in smaller units, such as milli-bitcoins (mBTC) or micro-bitcoins (μBTC).