When you take a deposit write a little contract/reciept out that clearly states the purchase price the deposit taken, details of the goods and any conditions such as finance.

Also state a date that the car is to "settle" by.

all parties sign the contract.

This means if it doesnt settle within in a reasonable time from the date set the deposit is returned if its your fault but kept if its the buyers.

both sign it and keep a copy.

Saves these hassles down the track.

I dont think a 1 day delay would void the contract you have. Tell them to buy it or lose their deposit. Offer to reduce the price by a days hire car if they get stroppy.

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Arguably my Automobile is Awash with Alliteration - Subaru Sti S202