-depending on sale price, cap rate will be in the 8.8%-9.3% range(rent increases each year).

If you want to introduce new rules or increase rents etc. they can simply withhold the rents and you go bankrupt.Not a good investment risk.

I did not deposit his check however, so it might not be considered a binding contract (no consideration was accepted).

If they are, you'll be restricted on increasing income and may even have to lower your rents if the utility allowances increase from year to year.Just a few things to consider.

Consideration must be exchanged for a real estate transaction to even be legal.  

I seem to recall a while back that there was also a discussion about sending your RECF portal a 1099-c in the amount that you are owed to take the expense (for your business), but this appears to have very similar considerations to form 8949.

Plus comparing that to investing in stocks.Based on what you just said, let's say the property increase by 10% every year. 

Unless you create one site/branding per property, that sells with the property,  which increases the required effort, but then not all buyers are interested in STR.

I was up front with what I was going to do, gave them fair option consideration and made their payment for them so they could move on.  

If you took that extra money you were paying down your mortgage with and invested it elsewhere, you should (hypothetically) be increasing your net worth much quicker.In regards to general savings, I have roughly 2/3 in the market and 1/3 in a savings account earning 1.3%.