Once you've bought shares you'll have a trading position. This will show on the Positions tab of the My Trading panel:
In the example above, you've bought 10 shares of the Patriots to Win proposition. To sell these shares you enter a new order using the Order Ticket - but this time to sell the shares:
This new sell order will be sent to the market to be matched - and when it is matched you will exit your position. The system always calculates a net position, so if you buy 10 shares and then sell 10 shares you'll left with 0 shares and no position on the outcome of the proposition.
If you sell your shares for more than you paid when buying, you'll be credited with a profit. But if you sell for less than you bought for, you'll be hit with a loss on the trade.