Mechanic on-demand to evaluate a used car

I'm not from the US but I have used kinda similar services before. Two kinds somewhat similar:

One is a service where the seller delivers all their cars to a local vehicle inspection service that goes through a checklist and any later faults in a part that was tested is under extended warranty. I'm not much of a fan since who the customer of the vehicle inspection site is can be argued both ways.

The other service is where the car gets delivered by the seller to a vehicle inspection site that is owned by a motoring association that the buyer is a member of (like ADAC in Germany for example). This service I have a high amount of trust in since they are "on my side", paid by me and will sometimes take dealers that misbehave to court (sort of like the EFF).

A service somewhere in the middle of these two I'm not sure I would trust. If I hire a local mechanic to look at a car for me how do I know he doesn't have a deal with the local seller? If he doesn't spot a fault will he pay to fix it when it breaks? if not, what am I buying?

In short: No, I would not use such a service unless it is backed by a trustworthy motoring association that is squarely on my side and who will go to court for or with me if needed.

If this mechanic is getting video from inside the cylinders with a borescope, doing compression tests on all cylinders, pulling diagnostics from the OBD interface, checking seals and joints, checking that emissions would pass in my state at least via OBD and signing their name on a form then I certainly would. ... especially on cars that pulling the plugs requires removing the fuel rails. I would also expect them to take it for a rough ride and see how it handles. Avoiding traveling cross country just to find that the vehicle is a pile of junk is worth much more than $125 in my opinion, assuming this system can't be gamed and assuming they are doing the above checks.

People do this all the time. There are companies that specialize in it. I've often had cars inspected remotely, but always by having the seller bring it to a specialist shop.

My experience with this kind of thing has been that it's hit and miss. It all depends on the person doing the inspection. Are you trying to do this as a startup? If I was convinced that the person really knew what they were doing, took lots of great pictures, scanned codes (including manufacturer specific codes), checked every panel with a paint meter etc. then I'd use it.

No, because on-site evaluations without access to a lift or all of the shop's tools aren't very thorough. It amounts to a visual inspection, which I can do myself.

It might be useful if I'm buying a car remotely and I need someone on the ground to look at it, though.

I'm not sure if I'm the right audience/user to answer your question but here's an offshoot thought: Wouldn't there be some value in building something like what you are suggesting but for electric vehicles only?