You have no recourse aside from suing them in a court of competent jurisdiction.
You will be very surprised to learn, however, what that is. Most likely it will not be one that is easily accessible to you.
And you have agreed to this when using their services -- because that's a contract. They might be in breach of the contract but if you are in Chicago are you going to sue someone in Cyprus or the United Arab Emirates for a few hundreds dollars? (These two are real world examples of where popular agents are domiciled.)
In very broad strokes, using agents really need to be avoided -- not only because of the above but because in case of IRROPs the airline can (and will!) deal more favorably with people who booked directly with them. Their assistance for those who booked with others very well might be telling you to talk to whomever you booked with. While others on the same plane are being rebooked at the desk to other flights, you are on the phone frustrated trying to reach an agent who might or might not be helpful and will lose time trying to reach the airline.
There are other issues with a third party agent. They might be using one carrier to issue all their booking (see my post ) and if that one goes under, then despite they are neither the marketing or the operating carrier you might not be able to fly.
To emphasize: even saving hundreds or thousands of dollars is just not worth the risk of not being able to fly.
Another possible issue is you might have two tickets when you thought you have one which might cause visa and quarantine issues.