Different business models...
Some are happy paying reg fee and pocketing a small profit, rinse and repeat, downside of this model is your creating a non stop job due to the low profit margin and you have to continually find new inventory to replace the sold pending the system is working and it's not as easy today to find good available domains as it was 20+ years ago due to more domainer competition.
I have bought quite a few reg fee domains over the last few decades but my business model hasn't been I paid $8 let's sell for $50 and repeat it's been more of a game of let's see what's the most profit I can generate out of $8. Beauty of starting from $8 or $10 is if I hold a domain for 10-20 years I'm out max $100-$200 so basically nothing and still basically all profit when it does sell.
You can hand the same domain to 25 domainers and all 25 will probably price it at a different price point. So I think the main variables that determine how quick something sells or sits...
1)Quality of Domain, most domains won't sell, there is serious trash registered, quite a bit fueled by automated appraisals that are worthless that make newer domainers waste their money.
2)Distribution, List everything Afternic, Sedo, Atom, SpaceShip, etc... ya never know where the buyer is.
3)Price Point, Might sell because domainer A quotes a reasonable price, Might not sell because domainer B shoots for the stars on a domain that never saw any form of a stars twinkle.
I'm not against flippers as profit is profit and I think it depends on your financial situation and where ya live. If I was struggling and $100 profit means something to me turn on hustle mode and let's get them 100's. In my case Chicago I generally don't buy any domain that I don't see at least a 1k+ profit margin on from an end user and 1k is rare usually looking 2k+ as $100 here is nothing. Since I'm old my portfolio isn't just the brandable you mention it's diversified not only on categories but also on generic keyword/brandable domains. So on the keep/drop question it's very rare I drop anything, this is more of looking at the total income of portfolio being profitable instead of drilling down into individual domains due to time constraints, I have dropped some domains but the percentage is kinda like yeah not really.
