There is also a matter of law in common law countries regarding deposits.

Goods are offered, accepted and any sort of cash changes hands and you have a contract. If you get real idiots there will be noise about this, so better to be clear.

Make statements in the offer (advertisement) like " Deposit of 10% to be paid to on purchase with balance to be paid inside 30 days. Deposit will be used for storage charges if the balance is not paid, will not be refunded and the bike will be offered for sale to other purchasers"

Clear language is better than any attempt at legal stuff.

On E-bay I always state that the auction will run to the end (i.e. don't bug me about buy it now), that collection in person is not offered (you aren't coming round to haggle or bring it back when buyers guilt sets in) and foreign shipping is not offered (I might change my mind on this now E-bay act as customs broker).

This will put some buyers off. It's the sellers decision if they are prepared to secure the sale by cutting into the margin by offering service after service. I often get customers asking if I don't want their business. I'm straight, I don't want their business on unfavourable terms and its better we all move on seek out customers/suppliers we are compatible with. If you depart on these terms the option remains to return when one party decides the deal offered wasn't so bad.

Andy