A sell transaction indicates that a good or service was sold and remunerated with coin(s). The price recorded is indicative(estimated) based on the market value when the sale 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 Sale -> Unclassified -> Trade.
You can auto use the auto-value feature for looking up historic value.
For more information on auto-value click here.