Then expand to building a portfolio of rental properties to build my assets and passive income stream.

A great start is by reading Brandon Turner's book on Investing With Low or No Money Down.The lady who sold us our first property wanted an income stream and not a lump sum so she could continue to receive her medical benefits.

I'm aware of some of the benefits of seller financing, such as a higher overall return and steady income stream but I'm wondering what type of drawbacks there could be to them in this situation (specifically an older couple living across country from the property).

The only answer really is to keep expanding, and keep improving operating standards till you earn more money than you need, can handle multiple vacancies, repairs, plus turnover cost all at once, while putting away cash for a rainy day, or find a 2nd stream of income to resolve any worries you may have.Since flipping sounds to be pulling at your heart strings, why not keep your rentals for a base, and go buy a property to flip.

He got waivers for being near wetlands , and a stream , has all the trees marked that stay . had everything staked out .   

As far as my goals I'm looking to do both flips (continue to build immediate working capital) and also build a portfolio of rentals for a more passive cash flow income stream as well.

What theyvwill look at is the property's income stream and if you've got enough experience to manage the property yourself going After closing. 

Or a stream of different friends of an owner who uses the apartment as a hotel room for guests.Co-ops generally do not approve of investment buyers.

You could opt to invest in mortgage notes, which would provide a stream of income and growth of your seed capital.The main thing is, you need the education before you put your money at risk, OR to partner up with someone who has the education and track record.  

It takes a lot of work and sometimes a good hunk of money to get a direct mail stream going.