Here's an example. A particular item (it's computer related for that isn't necessarily important here) is available from several sources. At the time of writing, it is £111.99 on Expansys, £117.48 on ebuyer, £119.46 on BT Shop. So you'd expect it to be cheaper on say, eBay, yes? You'd be wrong. The cheapest price on eBay is £126.31, with other people/companies selling is for excess of £130. Note that this is comparing like for like, and includes postage and packing. There are thousands (millions?) of things that are cheaper on eBay, but this isn't one of them.
Then there's the curious case of the price comparison websites that pollute the internet. You know the sort: you do a search for, say, how to replace your car's headlamp and you get one of the comparison sites advising that you can buy a headlamp for so much from a company you've never heard of. Many of these sites use so-called meta-data; they are not designed to provide useful information, simply to redirect you to another website regardless of what you have typed in (they receive a tiny payment for doing so). For instance, you type in "how do I replace a car headlamp?" into Google and you get responses such as "Book a holiday in how to replace a car headlamp", "Register the domain how to replace a car headlamp", "New medical breakthrough cures how to replace a car headlamp" and so on.
Google itself is a major player. Do a search on an item and there is an excellent chance that one of the results will be from Google itself, containing the line "Compare prices from £nnn at nn stores". If you do so, you'll obtain a list of results. And here's the strange thing: chances are that one of the results will be significantly cheaper than all the rest! For instance, if the average price for the item is £120-ish, there will be one entry that reads £100 (and it will be from a company you've not heard of). Click on it and you'll find that the price does not include VAT (meaning it is really £120 like the others). This is despite the fact that at the bottom of the page there is a disclaimer from Google that states "All prices include VAT".
A variant of this is where an item is significantly cheaper, but when you click on it you find it is not the same item as all the others. Let's suppose you have searched for a 25ml bottle of a particular perfume; the average price is £40 but one entry has it for just £25, so naturally you click on that. When you are taken to the retailer's site, you find that is actually for a 10ml bottle.
This type of thing is very common. We checked 20 different items over the course of a week. On no less that 16 occasions, the cheapest item did not include VAT or was not actually the item that was searched on. So what is going on? It's worth pointing out that the listings which are generated by Google are not paid for; Google do of course charge for advertising but such results are separate and clearly marked, whereas these results are supposed to be objective and based on a trawl of what is out there. One possible cause is that the listing mechanisms are faulty, or that companies have found ways to exploit them. Regardless of cause, the old cavet of "buyer beware" applies just as much in the online era.