Coin Collector Scam
Imagine someone tries to sell a perfume. It has a foreign name, and smells nice, so you purchase it for your mother's birthday, and at only £20 it's a bargain, and Au De Toilet usually sells for £55! (apparently) This might sound obvious but this scam comes in a number of different forms but always has the same core method. The Con-man convinces the mark to buy something that they don't know much about, but sounds like an incredible deal.
Originally a coin of significant value is sold to someone that knows nothing about antique coins, and as the owner needs a quick sale the mark can gain the profit. When they try and sell it, they find out it is worthless.
Don't think this would happen to you? How many things have you purchased that you don't have a 100% understanding of? Probably everything (I am fairly sure phones work on magic). You assume you aren't getting conned because you are in a shop, or trust the seller. So watch out, especially on sites like eBay. A seller will use several accounts to sell an item at an inflated price. When you search for the item, all the prices you see are inflated prices, therefore you assume that is how much it costs.
Usually under £200, but there really is no limit.
How to avoid being scammed
Being aware and always check multiple shops/websites for price comparisons and reviews.