It says they are blocked because they are in a country I don't post to.

I had one yesterday that tried to bid twice on the same item. Too many unpaid strikes. First one in weeks

In that case I wonder if the buyer has changed their account to a different region recently and not changed it back.

For example they moved to England for 6 - 12 months and haven't reverted their account to the Australian region again.

Often the changing of regions can cause these types of problems.