When I was a kid, my mom watched a TV show called "The Price is Right". The contestants that did the best on that show knew the cost of items such as a loaf of bread, a can of soup, a dining room table, that sort of thing. And if you think it's silly, then, quick, how much is a loaf of bread? If you do know, then there is no chance of you being cheated on price. If you don't, then look out.
If you're not an expert on the price of things, it's best to stick with a reliable retailer, such as Wal-Mart. But beware, even their online pages have outside vendors (look for the "sold and shipped by") whose prices can be anything they want them to be, and Wal-Mart is not responsible.
The most dangerous place to shop if you don't know the price of things is Amazon. If all you're looking at is "free shipping", then watch out for the product that normally sells for $4.99 being offered at $29.99. I've seen such ridiculous prices at times I wonder if they are marketing to people who don't understand a particular currency. Again, if you understand your currency, and the price of things, you will be safe. But imagine if everything was priced in yen. What can I buy for 1,000 yen? A car? A candy bar?
There is nothing illegal about asking too much money for something. I could put a "for sale" sign on my car right now and ask for a million dollars. Other than a few chuckles, and people thinking I was crazy, I'm not really doing anything wrong. But when something is priced wrong on the web, and you don't know what price is right, you could be hurt.