Recently, I've had dealings with a seller rjt17p based in Bristol, Avon who demanded a £200 non-refundable deposit within 6 hours of auction end for a 2002 Sprinter. As certain categories are not covered by Ebay buyer protection, this being 1 such category I would urge anyone never to agree to such a demand. For example, if you rely solely on the item description, which let's face it a lot of ebayers do and have never seen the item in person, should the item not be as described in the item description you will end up losing this amount. Once you collect the item you have purchased then there is not much that can be done as returns are solely at the buyers discretion.
In such circumstances it is best not to take any action on Ebay such as pay the full amount via PayPal, Mark Payment As Sent or transfer any amount via PayPal as a deposit or otherwise outside of ebay. Instead ask for the address and satisfy yourself that the item fully matches the item description before making payment and taking the item home. It is always advisable you do not take cash with you when you go along with you at first contact and always ask for a receipt or transfer any money via PayPal as evidence of purchase.
Of course the best thing to do is to view the item before bidding. However, as this is not always possible, if you haven't taken any action to make full Payment, Part-Payment or have marked Payment As Sent, in case of dispute and/or in case you're not satisfied that the item you have purchased fully matches the item description including things that have not been mentioned like odd noises, rattles, etc, then you can always opt for a Mutual Withdrawal From Sale and the seller will have a number of options at their disposal including second-chance offers, free re-listing, etc.
Never pay deposits, least of all non-refundable deposits and never fall prey to dodgy traders on Ebay.