… Without said systems built directly into the game itself.

Tons and tons of Gmod servers/communities operated their own cash shops and pay tiers for access to content on only their servers, its just that these were done on their own websites/forums, and then there would just be an integration layer between whatever db was storing that info, and the custom server game modes.

In addition, tons of gamemodes, with tiered levels of capabilities and/or assets, that allowed for others to try to jumpstart their own version of this kind of ‘business model’ were themselves sold for real money, again on third party sites.

(Often these gamemodes and assets being sold were acquired without permission to redistribute, and this kicked off a kind of arms race of people designing maps that would not function unless you had a specific gamemode as well, and then people would steal or reverse engineer gamemode code, and then more and more exotic and esoteric ‘anti theft/anti tampering’ methods would be used.)

Roblox just centralized all this into the game itself, functionally monopolizing it, and then greatly expanded upon it.