Originally Posted by
Grim Portent
Or Sheriff Bob could do what people normally did, rally a posse of locals, call on the neighbouring communities for help or bump the problem up to the local nobility or other higher level of government/law enforcement. Unless the land is wracked by plague, famine or civil war it should not take long to get a group together that can kill, capture or drive off the problem.
And mercenaries tended to be a near lifelong career, bouncing from war to war and either not working in between or becoming bandits for a while if they hadn't been paid properly. It's not the sort of thing where you fight for money only occasionally, you invest a lot of money in better equipment that allows you to charge more for your services, so time spent doing something else is time that doesn't justify your prior expenses. If adventurers are doing mercenary work they aren't adventurers, they're mercenaries. Though generally speaking mercenaries work in groups of more than four, so I'm not sure who would actually hire them.
Similarly caravan guards weren't actually a thing, travelling merchants either didn't need guards or were guarded by the local authorities in most places. It's the lord's job to maintain safe roads in their lands, it's not something to leave to private enterprise. Not to mention the general issues with letting landless and liegeless people wander around with military weapons and armour.