It doesn't depend soly on cashflow

Cashflow is one of the major reason for you to buy one rental property, but not the only reason.

Same thing for sell or keep a rental property.

Say it this way. If I can buy a property with all cash, of course I would have positive cash flow. But, I decide to buy it or not by paying myself the prevailing interest on the downpayment. If after paying myself the interest on the downpayment and I still have positive cash flow, I would prefer buying it.

The other considerations would be the appreciation potential, easy management, etc.