I'd worry that this could be a mileage broker. No details plus a totally unchangeable ticket, not just nonrefundable, would scare me.

You're better off purchasing tickets directly from the airline.

How do they do it? You might find yourself flying a third world airline with a dubious safety record, a circuitous routing, or a broken fare/separate tickets (perhaps using a LCC/ULCC that doesn't interline) so that you're not protected if a delay causes you to miss a connection.