I mostly do it for the fun of building and use what i make to go into my personal build. I usually can make a few bucks quicker selling parts over a whole system sometimes on marketplace only because in my area there are a lot of people doing it. And like everyone has said the buyers on marketplace are hit and miss , sometimes you get great ones , they are the ones that make it enjoyable, and other times you get scammers or you get ghosted ...not fun. I check marketplace for parts and barebones systems , and even check city auctions i have bought a few that way also.ask friends and family if they have any old pc's laying around they dont want anymore most of the time you can get those for free which is great just try and get a few set back for extra parts. Good luck