It's a terrible idea to plan on skipping a leg. The rest of your flights will almost certainly be cancelled. And it wouldn't be a short haul flight that could be replaced relatively cheaply, but a long-haul flight where a one-way home would cost a fortune.

Using an online travel agent is also a terrible idea. The airlines won't be able to do anything with your ticket until you are at the airport. Any schedule changes, cancellations, etc. would be a nightmare if you have to deal with the OTA instead of the airline.