In Guild Wars 2's case, there's a significant portion of the API made available to the public, including the trading post. https://wiki.guildwars2.com/wiki/API:

In Guild Wars 2's case, there's a significant portion of the API made available to the public, including the trading post. https://wiki.guildwars2.com/wiki/API:Main https://wiki.guildwars2.com/wiki/API:Main I don't believe this includes "write" operations like buying or selling, but since the in-game trading post communicates through the same web-based, it probably wouldn't be too much of a stretch to piece it together.