+ https://community.ebay.com/t5/Shipping-Returns/Shipping-to-Puerto-Rico/qaq-p/22221028
  Your items ships to US and Canada but not worldwide. 
  Puerto Ricans have two address options on eBay: 
  1. PR state and US country. 
  2. PR country I think your potential buyer is using address option #2 and is getting blocked. 
     Ask them to change their address to an US style address 
     and they will be allowed to purchase. OR you can exmpet them by adding them to your exemption list. 
     http://offer.ebay.com/ws/eBayISAPI.dll?BidderPermitLogin