Well I have been doing it for over 4 years now, pretty successfully I would say.
There is domain flipping and there's a domain flipping.
Some domains are valued high because of their domain names. Whether it's something super brandable or LLLL kind of domains, etc.
Then some domains are valued high because of their SEO power.
Both of these work. Domain name flipping however is a long run. You might have domains in your portfolio for decades and some will never get sold.
Anything with a nice name nowadays is either registered or is sold by registrars at premium.
Selling domains based on their SEO value is what I have been doing. However, the amount of domains you can get your hands on is limited. There's maybe what... 50 - 100 domains a week on all the auctions that fit my criteria of cleanliness and power.
Like most of the brokers who sell SEO related domains, we bid against each other and at the end of the day, it's just about who has a better and more willing clientele. Or who has more ignorant clientele, considering that I know the kind of abused crap some other brokers sell.
You can forget domain scraping. We have abused the entire Internet 3 times over. I used to process more, but nowadays it's maybe 4 million domains a month. You take a bunch of 4 million, run them through availability checkers and end up with maybe 3 million. You run those through Majestic, filter them out through various metrics, run them through basic filtering bots, hand check them and you end up with 200 - 250 clean domains with TF5+ RD10+ at best. Out of 4 million.
Any new domains that expire are either sold at the auctions for premium or are drop caught and then sold at the auctions for premium or picked apart by hundreds of SEO people via expiredomains.net and similar. Whatever is left is either completely hidden under 50 tons of manure or has somewhat escaped the clutches or people parsing expireddomains.net daily. So bottom line, don't bother.
What you can do, and actually make some money with, is drop catching. You won't ever catch some super high end, nor even high end domains. Impossible. But with one or two drop catchers you can get a few domains a day with decent metrics.
Ignore the history on these, make sure they are indexed or at least look somewhat legit, catch them and then sell them on auctions. There is enough idiots in the SEO field to buy them, regardless how spammed they are. I played with this a couple years back and even though the return is a bit slow, you will eventually make at least double your initial investment.
So if you decide to spend $1K per year, I can assure you, that unless you completely fuck everything you touch up, you will have at least $2K by the end of the year. Buy 200 domains for $8, sell 20 of them for $150 and you are good. You have a year to sell them.