This model is different from other investment models as it makes extraordinary deals available to ordinary investors.

They are ordinary income and you've likely got a bit of a struggle on your hands to work this through.You can absolutely set up retirement plans based on your flipping income, even if you're flipping with partners, but you're in a situation where you're trying to have your cake and eat it too.If you are really going to insist that your flipping is capital gains (it's not, but let's say it is), then no - no retirement contributions because retirement contributions are based on EARNED income and capital gains is PASSIVE/INVESTMENT income. 

In a seller finance does seller pay more capital taxes or ordinary taxes or both if they dont do seller finance?

I agree with Jason, but you might be paying SE taxes on top of ordinary taxes as ordinary income( not short term capital gains). 

Was looking at a small multifamily property locally where the seller had quoted this policy to substitute his ordinary maintenance expenses when I asked for his P&L on the property.

Way the worksheet is laid out, it will pull the applicable number to determine what bracket your capital gain will be taxed depending on your ordinary income level. 

If the IRS determines that your intent during the year was always primarily to resell (think like a long rehab and flip) then they can call the gain ordinary income and you would pay regular income fed and state plus maybe self-employment plus maybe the ACA surcharge - It adds up tremendously. 

If you plan to fix&flip then it will be taxed at your ordinary income rate.

And perhaps somebody can jump in to verify but I believe everything I've earned from Fundrise is taxed at the ordinary income tax rate whereas I'm showing massive paper losses on my passive investment due to accelerated depreciation.

I just found the financing a bit out of the ordinary and wondered why.