Are non refundable deposits standard practice when buying a property before the legal contracts are signed ?
It sounds like a bad bet from your point of view.
If you can't complete, then the developer keeps your deposit.
If the developer gets a better offer, then he or she can accept the better offer and return the deposit to you.
For the developer, it sounds like a great idea, it's a one way bet for the developer.
The deposit does not make it legally binding, your deposit means nothing if the developer gets a better offer.
My advice, don't pay a non refundable deposit, it does not secure the sale, and you lose the money if you can't complete.