Is there anything I need to know before selling pcs I build?

I'm fairly new to the pc hobby, I did a side build for fun and I figure I can post it for sale and keep it going.
Are there any niche things that I need to know about, tests I need to run, etc? If I'm going to sell someone something like their personal computer I'd like to make sure it's done right.
This is my second build this year so I'm pretty familiar with the basics of pc building, im looking for the juicy stuff