Amazon differs from other platforms as it is both the marketplace provider and a retail competitor within that marketplace. As such it has to protect it's own buying position with many brands and often works closely with them to ensure only sellers authorised by said brands are allowed to sell via Amazon. In turn that helps the brands control the stock offered for sale on Amazon to try to ensure it is stock intended for the UK/EU/USA (etc) market rather than parallel import/gray market.

Most marketplaces don't care who sells what as long as it isn't fake and they get their commission, Amazon are in a different position so you will need brand approval for some items.

Amazon also prides itself on being very customer focused which a minority of customers try to take advantage of so expect a higher level of 'item not received' or 'empty box received' claim attempts, switcharoo returns containing used/broken items etc etc than you would find on some other platforms.