Then your sale will be for $450K ish (you do subtract closing costs). 

You should get $2632.83.Subtract THAT amount from the principal balance.

Positive CF subtracts from it. 

I ask why and he stated that when the tenants were late on their rent he subtracted the late fees out of their deposit.

Should I include vacancy and maintenance in the monthly expense and subtract it from the cash flow, or keep the cash flow at the higher number (keeping escrow's for the vacancy/repair) when calculating ROI? 

If you don't want to do that, I definitely suggest that you have a CPA prepare this year's return as there will be some extra complications with the property sale and depreciation recapture.Essentially, you get the "credit" for that $22,273 in the year of sale as that is subtracted from the sales price to reach your capital gain amount.

Subtract the flat fee buyer agency fee of $6000, meaning the buyer is credited $6,500 at closing.

Net rental income about a couple thousand each month (they wouldn't give me a specific amount after they subtracted out mortgage PITI.

Will they subtract the $150k from the HELOC to give the credit line balance?

Megan Luckily the math part isnt too high level just addition, subtraction, some mulitplication and division but we as a community can all help you through it.Leave the complicated math for the wall street quantative analysts and derivative traders.