^ I'd agree with a lot of this advice in particular (though it's all very good)
I started selling used parts from bulk lots a couple of years ago. Although there can be a LOT of time spent on cleaning, sorting, researching and listing, there is a visceral thrill for me of digging through a bulk bin off Craigslist and finding those few rare pieces that can (sometimes) pay back on the whole lot. That, and having some inexpensive bulk used items (or rare items) in my inventory keeps things diverse and get people shopping.
Now, due to some personal / family things, I don't have the space at the moment to do all the work for cleaning & sorting the used lots. So I have been dabbling in parting out sets to help diversify my inventory. I still get to satisfy my OCD by sorting and organizing LEGO, and it keeps my offerings fresh.
Dealing with new parts has a LOT of benefits that have been mentioned already. So if I could say the one "drawback" of dealing with new sets / parts is that you really have a more straightforward understanding upfront on what the costs and profits should look like. Aside from finding good buying deals, you loose that "treasure hunt" and "goldmine" effect that used bulk lots can provide. That's a fun aspect of this hobby for me.
From a business perspective, you can build formulas and strategies around dealing with new sets / parts. And the good thing is that as mentioned above, there are still a lot of niche ways to do things.
Just my two-cents.