The main reason third generation processors are still quite expensive is because they are still relevant. They are still widely used in businesses across the world, due to their affordability and performance. Picture yourself the owner of a business that needs computers to browse the internet and make excel spreadsheets. Third generation processors have more than enough power to accommodate for the average person, for the simple and medial tasks.
Another good reason for the pricing of the processors, especially on a site such as eBay, is due to the original price a person paid for the processor, and the generalized pricing across the web. If you were to sell an i7-3770 on eBay, you bought it for $300 and you see other people selling it...