>"use Valve’s software" Not really that accurate; sure they access the valve servers, however most all of them use custom software made by reverse engineering

>"use Valve’s software" Not really that accurate; sure they access the valve servers, however most all of them use custom software made by reverse engineering the steam client called steamkit [1] and it's forks. Steam doesn't provide any public api's for trading. [1]https://github.com/SteamRE/SteamKit https://github.com/SteamRE/SteamKit