I guess it varies greatly when buying PC parts. As someone who has built for himself probably 20 PC's I have never had to RMA a part that was broken at no fault of my own (e.g. rushing a build, overclocking and other heat related issues that came about a while after installing the part) and certainly never anything DOA, at least that I can recall.
I also worked for a computer re-seller for a couple of years in my late teens and probably built ~300 computers of various configurations and the DOA parts were less than a handful (I am talking a single digit) and this is taking into consideration that we went through thousands of parts, I guess I am just lucky. Though I will say this, we did receive a number of returns of parts, and a lot of which we could attribute to user error.
Having said all that, I have gone through a bunch of RAM sticks, PSUs, a couple of motherboards and even a CPU and have experienced and fault diagnosed some of the most bizarre and fucked up situations. If you're not very PC literate (and in some cases even if you are), fault diagnosis and fixing can be an utmost pain in the ass at times and makes you just want to turn the bloody thing off (if it even turns on) and go play a console game.
When it comes to building a PC these days, it's very easy as long as you take your time with it (gone are the days of manually setting cpu voltages, multipliers, irqs etc). Do some research, take your time, use an anti-static strap, don't rush or stress, use those anti-static bags to lay your parts on, take your time and again TAKE YOUR TIME (I have broken things because I was in a rush before, try explaining to a re-seller that your motherboard CPU socket's pins were bent when you opened the box lol). But if you're at all not confident get the shop to do it, only costs a few bucks and you can have peace of mind until it breaks next
edit: I have to also add an advantage that you 'new' PC builders have. The Internet, there are so many guides, videos and forums for advice that if you do your homework you can build your PC with minimal stress and worry, depending on the person of course
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