When I was a travel agent, most airlines had their own booking system.
Since we were closest to St. Louis, we used PARS which was the TWA system.
When TWA went bankrupt, we went with the Apollo system which was United, I believe.
I heard that Sabre was very good, but was glad not to use it because it was American airlines.
American was notoriously hard to work with for travel agents.
That’s right, the system travel agents used was the same as the system the airlines were using in terms of booking and boarding passes, back when paper tickets were universal.
The travel agency I owned was a franchise, the franchise negotiated the reservation system agreements and override commission agreements that we used.
We had no choice, when we bought the franchise, it came with System One which was Eastern/Continental airlines, later we switched to Amadeus which was basically a follow on for System One.
When someone walked into our agency as wanted a ticket San Francisco or anywhere else from Jacksonville, Fl we could immediately tell them the lowest price and conditions to get that price.
If they were genuinely interested, we could look for availability for that price based on when they wanted to leave and arrive.
We were authorized to sell tickets on any airline in the world that had automated airline ticketing.
We sold tickets on every domestic airline and most international airlines.
International tickets paid a standard 11% commission, some would pay more if you could move volume to them.
We had one customer that had a lot of business in Brazil, Varig is their national airline, this customer flew their employees business class to Brazil, after selling a few tickets the Varig sales representative called us and said, when we booked a ticket to call him, he could upgrade the ticket to first class at no additional charge to the customer and pay us 24%, which was a nice commission on a $2500 ticket.