How can a home seller keep a buyer to completion?

In the current falling property market finding a buyer is hard enough but keeping that buyer to completion is quite a feat! Does anyone have any ideas to incentivise a buyer to stay the course accepting that a property has to be very realistically priced? I’ve had two ideas, cribbed from the house builders, but I think they need a bit of work on the detail for personal use.

1. The seller asks for a non refundable deposit to cover expenses in the event of the buyer dropping out so at least the seller hasn’t laid out for legal and valuation expenses for their purchase which they have to write off.

2. A significant cashback figure on completion.

3) Also to make the prospective buyer(s) responsible for the needless costs of the HIP's after they pull out from an agreed sale, or at least a large proportion of the cost. (Perhaps the Government may refund this to people who only give backword the once. ? ) after all it is designed to assist both parties.... so they claim.!

To be applicable once the buyer(s) have indicated they want to proceed with the purchase, by making an offer that is accepted by the vendor.