Fantastic question. Here's what I did, it took a bit of work to set up but every day I get seller only leads contacting me, it's completely automated, and happened totally by accident. Also I've never had to bother family members, cold call anyone, or do any other strategies that are not scalable.
Two years ago I got my realtor's license so I could get comparable sales data and close my own flip deals. Then I created a carrot website using a lot of Jerryll Noorden's techniques to rank high on search/SEO rankings for motivated sellers ie 'we buy houses' kind of terms.
Then something funny happened. Even with pictures of a nasty old house on my website, even with all of the keywords targeted directly towards motivated sellers, 80% or more of the calls I got were from sellers best suited for listing on the market with a realtor.
I'd chat with them, explain what they could expect for an off market offer vs on market listing price and the PROS and CONS of each, and in a lot of cases once I established rapport they'd say something along the lines of 'I think listing would be the best option after all - do you know a realtor?' And I'd be like well, ya.
From here I'm now in a position where I spend no time marketing, if I want more leads than I generate organically via website SEO I drive more traffic to the site via paid google or facebook ads and I can pretty much be as busy as I want to be.
Also no matter who calls me, I can help them. Retail seller and I'm already too busy? Refer for a referral fee. Motivated seller with a teardown? Wholesale to a builder / infill developer. Motivated seller with a home that'd make a good flip? I'll buy it and flip it.
Because I understand both the on market and off market options I can provide more information and more comprehensive service than most realtors or direct buyers that specialize in their own niche. In addition I can be completely objective when discussing options with them because I genuinely don't care which route they go (direct sale, listing, creative strategy etc) because regardless I can help.
Anyways Trevor Mauch is the only one I've ever heard put a name to it and he calls it the hybrid realtor / investor strategy and it's been incredibly effective for me once I really learned how to implement it properly.