It depends on what you're buying. For a single domestic flight in a few days, that's probably true.

But if, say, you're buying an international ticket, the sort where you might have to change the return date, then I'd suggest buying direct from the airline. When buying through an online agent, then sometimes any changes have to go through the agent, usually making the process more expensive and more cumbersome. Ditto a flight where you might not use the ticket and want a refund.