One free addon, four sections: DPS tells you what every item is worth for your spec, Professions finds an available crafter anywhere on your realm or levels your profession at the best price, Economy shows what your items are worth and the auction house deals, and Dungeons lists each boss's loot.
Interface\AddOns folder. You should get a BrokenMetaWeights folder./bmw).Yes. It is free, its code is readable, and it automates nothing: every crafter announce comes from their own click, like with group-finder addons. Players contact each other by whisper, themselves.
The game only lets an addon post your announce after a click from you. So your availability resets when you log in, and an announce lasts one hour.
No. The directory goes through the game, between addon players. Nothing is sent to the site unless you choose to share your data.
They need a recent version, your realm and faction, and to have clicked Available. Click Refresh in the Crafters tab.