eBay: ‘rare’ 50p coin promised to ‘increase in value’ is selling for £1,250 - why?

EBAY is a good place to find rare coins. If looking to purchase a commemorative coin, the site can be a good place to start with lots of options on eBay to choose from, and some can fetch a lot more than others. Recently, one ‘rare’ coin is selling for £1,250, but why is it so expensive, and is it worth it?