Vendor Mods

AlreadyKnown villiv

Colorizes items that you already know in default merchant and auction frames.
WoWInterface

GnomishVendorShrinker Tekkub

GnomishVendorShrinker creates a simple, compact vendor frame. GVS brings an interface similar to Auctioneer's "CompactUI" to the vendor frame.
WoWInterface, Curse, github

Peddler Cartas

Allows you to flag items that you plan to sell, automatically selling them later when you interact with a vendor. It automatically flags gray items you loot to be sold, but you're free to un-flag them.
Curse, WoWInterface

Reagent Restocker CobraA1

Reagent Restocker is a simple addon for automatically purchasing items, selling unwanted items, and repairing. Unlike many other similar addons, Reagent Restocker supports drag and drop and has a full GUI for editing buy/sell lists. Even though it may be more useful to some classes than others, Reagent Restocker works for all classes.
Curse, WoWInterface

Scrap Jaliborc

Have you ever felt tired of selling, over and over, all that items you don't need at the merchant? There are several addons that lets you create a "junk list" and sell all the junk in your bags with a single-click, but they are always hard to use and doing the simplest tasks, like changing their filters, can be a real pain. Scrap tries to fill this hole: it is designed to be an easy to use and intuitive addon, but lightweight at the same time. If you are tired of slash commands and hard configuration, then this is the addon for you!
Curse

Sell-O-Matic2 Silmano

The purpose of this mod is to help selling items in an easier way than just clicking all of them in the merchant. Uses filters and modules to set items to be sold or destroyed.
Curse

StealYourCarbon Tekkub

A simple reagent restocker addon. When you visit a vendor who sells an item you have told SYC to buy, it buys it for you.
WoWInterface, Curse, github
Review: WoW Insider

tekJunkSeller Tekkub

Simple addon that automatically sells all your grey items when you visit a vendor.
WoWInterface, github