Don't apologize, don't explain how the 'chosen' tenant was more qualified than they were, wish them well in their search.You're running a business and you've made a business decision. 

I'm the sole manager of the business and the properties.

Having the seller carry a second is not a problem for the first, as long as their business with you is finished and the loan has closed.

I just finished The Book On Rental Property Investing by Brandon Turner, and it made me even more excited and determined about getting into this business.

Definitely open an LLC if you haven’t done so already and once you have a couple under your belt, you will have lenders lining up for your business.

They were selling on pro forma, but the market and the debt market have shifted, and it's aa lot harder to achieve higher pricing with lower rents.We've purchased over 2,000 units, and the simple answer to the question is that they are either mom and pops and run their businesses with ZERO KPIs, they do not want to risk losing tenants if they raise rents, and they don't treat their asset as a business.When you take over, it can be difficult to get rents to market if they are very suppressed.

Quote from @Kody Smith: without getting too deep into my finances, what my strategy is...1. take loans to buy properties (leverage)2. fix/flip for day to day cash flow income3. use some of the fix/flip income to buy consistent cash flow through:- business acquisition, - commercial properties (MF and store fronts),I have no intention of doing ALL of the management myself, I will hire management as needed (medium term goal)my expected core role is money provider, evaluating the numbers, making decisions, and connecting with investors, wholesalers, lenders, contractors, and property owners.while I am just on step 2 of the journey, I am looking toward step 3 to stabilize, and not worry about if house prices drop mid project or not (or not worry as much) So to be fair this is more about obtaining and growing a real estate business not simply buying rentals for cashflow.

All too typical park owner who took all the profit out of the park without reinvesting in the business then wants to sell you the potential he was too cheap to invest in. 

If the proforma has the rent roll at $1400 a month, there is a discrepancy that could lead you into a bad assumption on your business plan.Also one caveat, be aware of physical delineations around the area like bridges or railroad tracks or a canal. 

For example, it is not unusual for me and my business partner to buy 4-unit buildings for about $250k and be able to rent them out for $4,000+/mo.