I think the latter part of that is a huge jump. How is seeing a billboard for a plumber promoting bad behavior?
Even then, you've probably not picked the best plumber this way.
A better question would be how is seeing ads for sports betting in between periods of a game promoting bad behavior.
And the answer to that is very obvious -- the legalization of online sports betting and the nonstop barrage of ads for it on tv and social media is bad for individuals and society as a whole.
Are you suggesting to wait around the hardware parking lot to hope to hear about someone who needs a plumber?
The plumber would dedicate a raspberry pi or yesteryear's phone with the bad battery or cracked screen, and they'd leave it plugged in in a closet somewhere and configure it to talk to a few peers who they know personally and who will vouch for their legitimacy. They of course would be vouching for others in the same way.
Nodes would gossip about services that are available, so you could figure out which plumbers are nearby and which of your peers trust those plumbers. Since you're operating on a web of trust, you can find a mutually trusted third party to act as a mediator in the case of disputes, and if you have a good or bad experience you can also gossip that info to your peers so that they can aggregate a sort of review system. But unlike Amazon's, it's full of people that you know and explicitly trust, so it's much harder to game since there's no single source of truth to target/abuse.
It would exchange the kind of information that traditionally comes via ads, but I think it would be so much more effective than ads because all parties want it to succeed--whereas ads have carved out a rather hostile landscape for this sort of thing.