I don't really know why some users ignore PMs relating to accusation, but later on when things are getting serious you see them show up with some excuses/reasons/defense.

A simple PM reply stating that the address they send btc to is an exchange or a corporative address (if something of that exist), would have solved this problem.  Now, you have made it public, I believe he will have to reply. Maybe he is waiting to see the forum reactions first before he will clear the air.