Is there a way (plugin or something) to make bot sell these items while visiting merchant ? Thanks in advance for suggestions.
I highly recommend using a WoW addon - Trashcan - Bags & Inventory - World of Warcraft Addons - Curse
My addon of choice for this is AutoVendor - you can /junk [itemlink] to add items to automatically vendor. It has switches to auto-repair (from guild bank if you want) and can be set to automatically vendor both items you can't use and items that are sub-optimal for your class (e.g. cloth items for death knights).
Sell-O-Matic is another addon that does all of the above mentioned with sell lists, delete lists and save lists. I like lists that I have total control over, so this one works for me. Very little is assumed or automatic (except greys which are sold automatically). I have to specify what to sell or destroy. I'm a control freak, what can I say. Some of the above addons make assumptions about character class or spec and sell/destroy based on that. I'd prefer to have the choice. Just my $0.02
Tiny addition to above information: Make sure you SELL the things, don't DESTROY them. They're actually worth a pretty penny for a free side-catch.