Also as, machinestops said, you can get the Framework laptop prebuilt.
As for the repair shop, kind of sounds like they just didn’t want your business.
I said there was no point in doing it, not that it can't be done. With a top Elonesque mind such as yours you should probably know the difference. Tips fedora.
The situation you described does suck, but I've built several machines, often from cheap used parts, and been lucky enough to never run into it. Most of the time self-assembly makes sense, you can't really "do it wrong" easily if you pick compatible parts and follow manuals. If it doesn't work getting used RAM stick or PSU for testing is usually cheap, if those don't fix it then it's probably either CPU or motherboard. Any computer repair shop should probably have those lying around anyway.
A few years ago I bought a new PC by buying components. When I put it together it didn't work, I took it to a computer repair shop and then couldn't figure it out either. Not knowing which part was broken or if it was something I did wrong, I ended up throwing everything in the trash.
Since then I just pay the $200 for it to be assembled and get 5 year warranty. IMHO self-assembly makes no sense unless you're a youtuber earning money from making videos doing it.