I guess I'm one of the few that don't buy from other sellers. Mostly because I don't know what environment that they're mixed in. Are they mixed in someone's filthy living room, kitchen, or bathroom? Or are they mixed in a sterile lab? Who really knows for most of them. I started mixing my own liquid after a week or two. I now mix it 10ml at a time using the same processes, for both manufacturing and cleaning/storing of equipment, that I used when I worked for a pharmaceutical manufacturer. Some times I'll mix a few on the side into amber glass vials that I know are better steeped, and then let them steep for a few days while I vape on stuff I make every other night. When the other ones have steeped, I roll through those one-at-a-time before mixing anything else. This way, my liquid is always fresh, and my ldpe bottles don't have time to really break down and substantially leech into my mix.
The other part of the reason I make my own, is because I can come up with flavors that are unique to me, and I can share them with a few friends I've gotten into vaping recently, and use their input.
It's an ever evolving process for me, and will surely continue to be.