If the item you are purchasing is for a small amount (including postage) their feedback is above 99%, you are most likely to just bid/buy the item and be done with it.
If the item is expensive, first of all check if you cannot find it cheaper somewhere else.
If you are based in the UK goto www.froogle.co.uk, the USA goto www.froogle.com enter what you are searching for, click oln price low to high , and you might be suprised to find the item at the same or lower price on a website
(obviously you might need to check that the web shop is genuine)
I’ve bought expensive items on ebay but have always made sure that the ebay seller has sold a lot of items with good feedback. You can always contact the ebay seller, maybe you’ll even go and see the item before you bid on it.
If the item they are selling is not what they normally sell, again be a like bit cautious,.
Be careful not to feel that an item is genuine because someone else has bid on it, it is very easy for the bid to be from the seller under a different user id(this is illegal), or a bid from someone who is not use to the ebay system.