The story
When Atari first launched the Atari 8-bit computers in late 1979, the company kept most of the hardware details secret. It intended to be the primary supplier of software for the platform, as had been the case with the Atari Video Computer System console. By the end of the first year on the market increasingly sophisticated applications from outside Atari were nonetheless becoming available. There were, however, a limited number of distribution channels at the time.
Dale Yocum approached Atari with the idea of setting up their own third-party publishing arm. With Atari's distribution capabilities the products would be seen by many more prospective customers, and at the same time, Atari would make money with every sale, money that would otherwise be lost. Chris Crawford later stated:
The guy who cooked up the idea, Dale Yocum, was trying to explain to the management that there are a lot people out there that like to write programs and if we can publish these programs for them, it's a win-win. He put together a business plan for it and said 'Look, we only need a little bit of money and this thing can be self sufficient and it might make some money.' They grudgingly agreed to let him do it because the Atari platform desperately needed a larger software base, a void not being filled by the other publishers of the day. And so he did it and very quickly made it into a monster success. It was a major profit center for Atari. They rewarded Dale for his initiative by bringing in another guy to be Dale's boss... so Dale, in disgust, transferred to the new Atari Research Division under Alan Kay about a year after APX launched.
Catalogs
Atari mailed catalogs to all computer owners who sent in warranty cards. The first issue of the catalog, Summer 1981, stated:
Atari offers a wide variety of useful and entertaining software... we've come across other interesting software deserving public recognition... [APX] will make such software available quickly and inexpensively... We'll keep costs down [by using] simple packaging and we'll rely on user-written documentation... What we'll offer, then, is a lot of interesting software quickly and inexpensively.
The quarterly publication included descriptions and screenshots of each program, and advertisements for computer magazines. Many APX programs were games, but it also distributed applications, utilities, programming tools, and educational software. A few non-software products were also sold: the book De Re Atari, ports and connectors for the Atari 8-bit line, a kit to expand the Atari 400 to 48K, and a cartridge with EPROM sockets.
Discontinuation
According to Atari CEO James Morgan, APX was losing money in its mail-order business so that part was shut down:
Moreover, Atari had to come to grips with the fact that Atari is not in the mail-order business. However, APX will continue to review products sent to Atari by outside programmers. If the programs are topnotch, they will be added to the main Atari catalogue. Otherwise, they will not be sold by Atari in any fashion.
Read the full text
The final catalog, dated Winter 1983–84, was much smaller than previous catalogs. After the discontinuation of APX, Antic magazine published some former APX titles, such as the games Galahad and the Holy Grail and Dandy, as "APX Classics from Antic". Antic continued soliciting new submissions as Antic Software. The Antic Software catalog was bound into issues of the magazine and later included Atari ST products.
From Wikipedia’s article on Atari Program Exchange, shared under CC BY-SA 4.0.3