Getting unlucky: How I lost 100€

About 2 years ago I attempted to build my PC during summer holiday. Someone told me which parts to buy, I then got them and carefully assembled everything (was the first and yet only time I did something like this, but I've got some electronic background, so I thought it wouldn't be too bad).
Then surprise, it didn't boot. I asked around, took most of the stuff out of the case and trid to find out what'S wrong with a friend on a skype call. Conclusion was "it's likely the motherboard". Before sending it back I took a closer look and found 2 open contact holes, likelyfor the BIOS battery. Send it back with all cables that came with it and as the seller wasn't generous he told me that I'll only get a replacement, if the manufacturer found an error.
2 weeks later I got another board and made a small setup for testing without case, external graphics card etc. Again not booting, same holes opened as the last one. Send it back with all the extra cables again.
This time it came back after 4 weeks (which was the maximum time they have given for such a thing). By that time I was about to throw away the whole damn project, I was quite frustated with this endeavor. But this time there was a proper contact where the holes were left open on the other two, the thing was working as intended and the seller wanted me to pay him, because the manufacturer "couldn't find any fault". Also they forgot to send back the SATA-cables again so I couldn't even get the stupid thing to work. And the holidays were about to end.
Ended up contacting the seller, telling him about the missing cables, that the battery wasn't connected the first two times and asking why the heck the manufacturer needed 4 weeks to confirm that his product wasn't working. They send me some cables while stating "this is not how we usually do things", I just didn't pay the extra money and never heared of them again.