Quite possibly...
Some established providers, ie AllGerman/WIM/Nigel Langs etc, are pretty obvious and any others could be on a submission basis, via the test zone. Any submission would sit there for a week and would need positive membership endorsement of say 50,000 post counts to be added to the endorsed provider section, (the 50,000 posts being the combined total post counts of the members endorsing each provider). This would provide reassurance to members that those providers are capable in what they are providing and that people have used those providers previously and been happy with them. Admins would have the final say on any provider, and could reserve the right to refuse/withdraw a submission based on corroborated negative experiences discussed via PM, rather than open defamatory comments/accusations. The idea being for this to be a positive system.
Once in the Endorsed section, listings could have a finite lifespan, after which providers would need to resubmit.
For example, summat like: "Hi, my name is Al. I am Sussex based and provide the following services: Brake work, general servicing (except cambelts- I am able to do my own but there are better people for this job), bolt on panel replacement, hbv/thermostat/radiator/condenser changing, clutch/gearbox changing (with assistance), interior repairs/retrofits. Anything else considered, just ask. Will travel for fuel costs." could be posted in the Test Zone.
People for whom I/A N Other provider have carried out work for could then post their endorsement on the same thread in the Test Zone, and provided the combined total post counts of the people endorsing reaches the threshold within a week, then the listing gets moved to the Endorsed Section, where it will remain for a fixed period of time.
Each provider could perhaps be rated by members each time they use them
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but suspect that might complicate things
and this happens to an extent already anyways...
Apologies for rambling slightly, this is simply an idea which might be positive if it's workable, but equally is no matter if it's not viable