is there a particular reason this buyer is writing low ball offers i.e. are they seeking a "deal"? believe they can only afford to pay x amount dollars.
It does not appear they truly motivated or have to purchase. I would sit down them and have a real candid talk, and let them know your time is money.
In his particular situation I would suggest you charge a fee up front and let them know it will be refunded upon purchase (i.e. writing full price offers too) and if they don't you keep it. if they decline you know they are wasting your time.
best of luck