Why can't you contact eBay?

eBay Help is at the top of every page

CS staff are not going to make a seller sell to you (or anyone) though

There are many reasons this might happen

Seller have blocks in place for non-payers

Not sending overseas (as in your case) did the seller agree to combine postage overseas?  (because they can't due to how it's set up)

Not posting to PO Boxes

Some sellers have limits on how much a buyer can buy from them within a certain period of time 

How long a person has been on eBay has nothing to do with anything, there are no blocks for buyers who have been on eBay a short time, nor are buyers who have been on eBay a long time excluded from any blocks a seller chooses to have in place