I have seen it too many times where an investor will buy an older property or even a new build, they net cashflow similar to you for a year ($100-$200 per month) and then a $5,000 repair or insurance/tax hike here hits them and it's a loss - they either get forced to sell or are majorly setback and unmotivated. 

With one duplex you should be focused on selling 99.9% of the time and .1% on the duplex.

The seller was selling to the tenant living in the property.

My question is: when wholesaling, after finding a great deal, do you need real proof of funds to the get the property under contact or is there a way to get it under contract, find a buyer and complete a double closing/assign the right to sell without actually having the cash for the property? 

Nobody in their right mind would sell at such because they simply don't have to, there is no shortage of buyers happy to out bid your desired landing. 

If you want to mitigate all risk, sell your real estate, and hide the cash under your pillow, and don’t get out of bed ever.

Weird layout houses in a questionable neighborhood (making an assumption there) don't sell for appraised value.

These companies selling products tend to drain agents dry at these conventions selling every do dad that is supposed to give them an edge over everybody else.

All things considered, on the market for months, its now winter time, cold weather, holidays, and dark at 5 pm, not prime selling time, I offered $250,000. 

He is getting calls from clients wanting boats ready as soon as possible because they are selling them .