That's because those servers are using a modified version of the ZR plugin. They probably had developers who implemented the quick buy feature (using sm_<weaponname> or !<weaponname>) into the plugin.
That plugin that Franc1sco recommended also works but hasn't been written with all the ZMarket features in mind.
That plugin is good for the beginning (it also checks for infected players to prevent exploitation etc. iirc).
But if you really want to have the ZMarket features included (Loadout Save, Auto-Rebuy)
Morell, Server Manager of Mapeadores (?), has a github repository of a modified ZR Plugin that has those features.
Have look:
https://github.com/JMorell/sm-zombiereloaded-3