The cost of Microsoft Office from Microsoft or a licenced reseller starts form £120 for the home and student edition (or is via a subscription to Office 365). Any of these "licences" sold for a few pounds on eBay or Amazon clearly fall into the category of if it looks too good to be true it probably is. They may well install and work OK initially but they are certainly not legal, they will probably be some form of OEM or volume licencing key that were intended for use by computer manufacturers or large businesses and if Microsoft become aware that the keys are being sold illegally they will block them and your copy will be likely to stop working.