If they have less then 10 posts, they can't reply. They can post/reply on the general forums and place an ad but are blocked from messaging back to answer questions till they get the 10 posts.
Other thing to do, check by clicking on their name, when was the last time they were even on this site? Some people place and ad without even reading up on how this site works and have no clue or it's a bogus one-time listing.
What is the item you were looking at?