Imagine if you are selling on ebay. You will pay extra attention for any oversea purchase, because if the buy is not happy, it will be very hard and time consuming to solve any problem without a direct communication channel.
Plus a lot of online scam come from oversea.
When a seller is very concerned about those risk, it is understandable why he requires a phone number. Besides in some countries, Post do require phone number for both sender and receiver.
Of course as a buyer, you may not want to get calls from them. If no one compromise, you can contact them to cancel the transaction.
