The purchase transaction is for when you purchase a good or service with coin(s). The price recorded is indicative (estimated) based in the market value when the purchase was made.
On this screen it is possible to unclassify existing transactions; this will preserve the main information of the transaction and will allow you to reclassify it as a different type of transaction without needing to re-enter it. i.e. Make Purchase -> Unclassified -> Trade.
You can auto use the auto-value feature for looking up historic prices.
For more information on auto-value click here.