The DOT rules governing cancellation applies only to carriers, not to others, including the opaque third-party sites.

As with any significant expense, you should carefully read the contract terms before purchasing. Some sites even offer cancellation policies for some types of purchases and not for others.

Dirt cheap fares are often generated through Consolidator fares in which someone such as one of these sites has purchased in bulk and then resells. Nothing wrong with the concept, but the tickets can be wholly inflexible. E.g., they can't be changed in any way.

A great deal for some. Just read the terms carefully.