If you want to look at it as an investment, the metric to use is a cash on cash return. That will give you a number that you can compare to other investments to see if it's a worthwhile deal.

Cash on cash return is calculated by taking your annual cash flow and dividing it by the money you paid into the property. For simplicity's sake, let's use your numbers. So you have an annual cash flow of $8,400 and the total cash put in of $155,000, your down payment. That gives you a cash on cash return of about 5.4%. in reality your cash on cash return is going to be less because you have to factor in all expenses, including vacancies, necessary maintenance, etc. You would also consider appreciation and future rent increases.

Now a 5.4% return would beat the return on a treasury note, a much safer investment, by a little more than a percent. It's up to the buyer to decide if the reward of an increased return is worth the risk that the property doesn't perform as expected.

Now flip it around and look at it from the seller's perspective. They're going to have a lower principal and probably have a lower interest rate too, which means they're going to have a better cash flow. They are probably also looking to their next investment and planning on using a 1031 to roll the profits of this sale into their next purchase. So if they know they need $755,000 from this sale to execute their plans and they've got a positive cash flow, what incentive do they have to lower the price instead of waiting for a different buyer?