There are three major marketplaces: Ebay, Amazon and Aliexpress. They have their own payment system that's their priority: Paypal for Ebay, Amazon pay for Amazon and Alipay for Aliexpress. Logically bitcoin doesn't seem to look like their friend but what if demand is huge? Real answer is that demand isn't that good to implement bitcoin or any other cryptocurrency and at the same time it's not easy decision and when you are a huge company and want big to implement something big (yeah, bitcoin is payment way and amazon has a lot of transactions daily), you have to keep things legal and fair. Government's answer on facebook's libra is something that major marketplaces have to consider if they think about cryptocurrencies. There should be other plans too, for example how to do refunds, how should it be done? How will we verify users' bitcoin adresses? How will we get rid of money laundering? And many other questions that they have to answer. In overall pros have to outweigh the cons.