Your input is greatly appreciated.Multi apartment buildings, around 100 units total, 80+ 1BR/1Ba, 20 2Br/1ba, laundry, parking, pool, clubhouse, the usualsAverage apt 520sqft, $500 rent, $250 deposit (current numbers, about fair for the area)Not a great area, "C" grade apartments, 80% occupancyBuildings in foreclosure, sold by the bank - i.e. no solid records on maintenance , income, expenses, turnaround, etc.

In working out the details of buying a MF, is it normal to ask for the security deposit the tenants paid?

In fact, they will likely ask for a copy of your bank statement and documentation for any recent large deposit.  

The buyer said they will deposit the earnest money the next day.  

Increase the security deposit while you are add it- 2 people is more wear and tear on your unit.Kelly

On doing the deposit, the bank rejected one of his checks.

If qualified require deposit...You would be surprised at the number of people looking to move without any$$$5.  

Keep showing the unit until you have made your offer to rent it and have a holding deposit or signed rental agreement in hand.

He has no intention on paying any money back and will change his number after the deposit is made.

And that you handle like you would anybody else - bank statements, extra deposits if you're concerned etc.