We used to insert the buyers email as the "Reply-To" email, but some email clients either sent the reply to us (because they ignored it), or blocked/sent the message to junk/spam. Some email clients see emails having a different "Reply-To" and "From" address as a security issue, so our email system has changed slightly.
To ensure emails go to your Inbox, we set up a new email address JUST for emails being sent via seller listings (firstname.lastname@example.org). By having the "From" and "Reply-To" set as the same address, more emails should reach their intended destination!
Its unfortunate that you cant simply hit your emails reply button any more, but the new system is just as easy!
When you get an email through your listing it should look like one of the 2 examples below. If your email is HTML, you should be able to click the "Reply To Email" button to open a new message pane and reply.
If this doesnt work you will need to copy the buyers email into a new message and reply this way.
If you get the PLAIN TEXT version, there should be a link  towards the bottom which should reply as above.
Added in Seller FAQ on 15th Mar 2016