Both answers before me say 'show the list because it might confuse users'. That's for the most part true because it's convention and keeps the process linear. @Pdxd says that if the product isn't what they are looking for, they can easily change the search query or the filter.
I like to disagree and say show them the item immediately.
A search query that results in just one item is normally a very detailed search query. Like in the example of E-bay with the battery for digital cameras. Such a detailed search query tells you the user know exactly what he or she wants. Directing them directly to this item seems logic and shouldn't cause confusing because they already found what they are looking for.
If it isn't the item they're looking for they can still easily change their search query. All ecommerce websites I know of always have their search bar visibly on screen. Even better is the fact product pages often come with similar products, products other people that looked at that product looked at, products of the same brand, etc. If a search query is so precise it only results in one product and if this product is not the product they're looking for, the search query itself is normally fault for just a small part. There's a big chance the right product will be in those similar products.