You made the right decision not to respond to the message, as you should never respond to these type of messages if you receive them. The message you were sent has all of the obvious hallmarks of a scam - ie: buyer registered oveseas, the buyer's asking for your Whatsapp account details to make you an offer instead of using the Best Offer function on the eBay listing specifically provided for the purpose of making a legitimate offer, not to mention sending you his/her telephone number via eBay messages without having committed to a purchase. If eBay were to see that message then you could end up with your account being sanctioned as eBay would probably accuse you of conspiring to do an off-eBay sale. Therefore, do as has already been advised and add the buyer's User ID to your Blocked Bidders List. You could certainly do without the aggravation that this potential "buyer" could cause you should you decide to respond to the query and try to negotiate a deal.
