I heard that a loan isn't taxed on the income it produces until the loan is paid.

What a nice feeling to have these income producing properties and no long-term debt under them. 

It provides a steady cash flow for retirees as long as they produce reasonable returns than putting money in the bank.

Will produce a higher cash flow due to me not having a mortgage to pay.

When you buy all cash, having too much equity, you are basically buying your cash flow and the property itself is producing nothing.

The 10 year mortgage would produce on a 50k investment selling for 90k at 10 percent 32% ROÍ and a total return of 161,468 over 10 years.

A bathroom fan is probably going to produce very little, if any results at all, and it is never a good idea to vent anything into your attic -dry or otherwise. 

What hurts the return is how minimal the leverage was.Interest rates have risen substantially but are still low by historical standards.I suspect your best option is dictated by what sort of appreciation you expect going forward.  50% appreciation in the last 5 years is pretty good but, due to your level of leverage, it did not produce a substantial ROI. 

Many times I read that going direct with sellers produces some of the best deals, but what if it's listed with an agent and things seems to be taking longer than desired? 

A guy you pay $15.00/hr to knows almost nothing through they may appear to by the way they hussle and swing a hammer but trust me a trained laborer will produce good quality work and a lot more for you.