I paid him 20 k deposit and then paid him for all work once it was complete.  

He is asking about the security deposit.   

As soon as a tenant gives notice, I copy the template and create a new checklist, assign all the tasks (in your case they would all be assigned to you unless you have an assistant), and then set deadlines for particular steps like "Tenant Turns in Keys" or "Refund Deposit." 

Meanwhile he's going to be wanting his deposit back. 

This is unless you are allowing your buyers to go under contract and conduct full diligence which I can't imagine you are allowing since it would put your deposit money at risk, correct?

But how do you know what to take out every month from the bank account once the rent checks are deposited? 

You want Tenant A out, you inspect, you handle the deposit, then you move Tenant B in.

As a seller I would counter your “certain to close argument” with “great, I want a $20k, or higher non-refundable earnest deposit.”

Is that 5k left to pay on top of their deposit amount?

When I'm on vacation, grilling is often part of the plan, and having to pay a propane deposit and a $50 cleaning fee could be a deterrent.