You should see some of the AR15s I've fixed before.
I'm all about buy once, cry once and give that advice when I can. But I don't expect everyone to carry that same philosophy. But for crying out loud, you can only put so much lipstick on a pig. And buying a second rate AR and slapping the most inexpensive made in China tactical goodies you can from Cheaper Than Dirt on it is not going to improve performance.
One carbine I fixed up was during the great run of 2013. He bought it from a guy "who custom builds" and couldn't figure out why it wasn't running right. So when I was able to finally get the BCG and charging handle out of the upper, I found out why. Other than missing the receiver plate that's required to properly align the receiver extension and lower receiver, the RE didn't screw in far enough to cover the bolt catch detent. And as such, the detent traveled into the BCG jamming the entire system.
Plus a loose gas tube from an el cheapo gas block that wasn't quite tight. And how did the rear takedown detent and spring stay in the lower receiver without a receiver plate? Superglue fixes everything... And the flash suppressor that wasn't timed and I was able to break free with fingers alone. The FCG pins that were out of spec and would come loose just by cycling the actions. And how much did this POS cost said individual?
$1200. "But it came with three mags!"
I'm all about learning to do things. But do them right the first time and use good parts. And know WTF you are getting into before you start buying a "custom AR15."