The last time I bought concert tickets it was through Vivid Seats. The last NFL game I went to was through StubHub.

Its my understanding that some NFL actually has an official deal to sell through Ticketmaster first - though presumably any of these other companies could have that deal if they paid the NFL more.

FWIW I do think ticket convenience fees should be eliminated (or made so they have to be listed upfront) and this would be good for transparency - but I am also pretty sure that doing so would ultimately just raise listed prices.