most email accounts let you have 'filters' for incoming emails (sometimes found under the 'options' menu).  This will let you filter the emails from this bad seller rather than blocking emails from them.  Take a look at the emails coming from the seller.   Don't open them, just note who they are 'from'.  Set a filter to put emails from them into your trash/deleted folder.  The email I use has filters that include terms like "From name contains: XXXXX" and you put the name into the box.  You won't get to see them unless you look at your trash/deleted folder contents, and most email accounts empty trash/deleted folders every two weeks or so.'