The amount of gain or loss is equal to the difference between the sale price of the bond and the holder's tax basis in the bond (the amount the holder paid for the bond originally, including any additions to such basis).
Thus, if a holder purchased a $5,000 face amount municipal bond for $5,000 and then sold the bond for $5,200, the holder would have a capital gain of $200. Typically, the purchase and sale price of a municipal bond includes the dealer's markup; however in cases where a commission is charged, it should be taken into account by the holder in computing gain or loss.
In some cases, there may not be any capital gains or losses, and oridinarily they are minimal.
There is also the question of whether or not AMT will apply.