The purchasers in a private sale have changed their minds?
Answers:
If there was no written agreement its a dodgy area, they changed their mind, so you could claim the deposit for the hassle .
The law would side towards you, but they could take it to the small claims court saying you never said the deposit would be retained.
Like i said, a grey area.
Other answers:
you keep it. It was "earnist moneys" so you keep it
you keep it. It was "earnist moneys" so you keep it
Even without a written document, they still made a verbal agreement and the deposit is yours since their decision to not purchase delayed the sale that you could have made to another person.