While every commoner has the right to set up a business of their own at a place of their choosing, almost all of them join a guild to do so. The guilds represent enough economic leverage to hold against most nobles, and are able to offer protection and aid in times of need. Of course, most guilds operate like protection rackets, and everyone not joining them will find his business plagued by inexplicable misfortunes.
All guilds work together closely, and sit together in the Guildsraat of the ten most important umbrella guilds.
To put it shortly - Guilds try to cheat on everyone at everything and succeed about as often as not.
But, as already mentioned with tax fraud, this is true only for small ‘inaccuracies’. Big rip-offs usually end up with a lot of dead people on all sides involved and thus are considered bad risk-to-reward ratios.