Under @Torstein input and @teebling suggestion, this thread wants to be a point where to collect useful informations about special/hidden vendors in Azeroth! If we gather enough contributions we may decide to transform it in a full fledged guide!

A hidden or special vendor is:

• A vendor found outside any major enchampment/city;
• A vendor found inside an enchampment/city but sells something unique (for example recipes).

We will divide the list by zone, every vendor will be listed matching the faction that can interact with him ([H] horde, [A] alliance, [N] neutral)

If we are able to compile something comprehensive it will be a very useful tool for us :-)

Please comment this thread with vendor you know or with additional interestin details that you know about an already listed vendor (for example "this sells a recipe that is very important for etc. etc")

Personally I find very entartaining to find this vendor while leveling, even randomly, so it's not by all means a complete list (I just wrote the ones that come out of my mind)
If you don't want to spoil yourself the location of the vendor just don't click on the link. DO NOT WRITE THE COORDINATES IN THE COMMENTS.

Alterac Mountains

• [N] Bro'kin is the only source for Recipe: Frost Oil

Arathi Highlands

• [A] Deneb Walker is pivotal for alliance first aiders, since is the only vendors for for Expert First Aid - Under Wraps, Manual: Heavy Silk Bandage and Manual: Mageweave Bandage

Azshara

• [N] Jubie Gadgetspring sells engineering supplies and a specific recipe, Schematic: Deepdive Helmet

Badlands

• [N] Jazzrik, a blacksmithing supplies in the middle of nowhere

Desolace

• [N] Vendor-Tron 1000 spawns at the end of a timed event related to caravan kodos and sells a lot of supplies and many cooking recipes!
• [N] https://classic.wowhead.com/npc=12246/ spawns at the end of a timed event related to caravan kodos and sells a lot of supplies and an enormous amount of recipes!

Dire Maul

• [N] https://classic.wowhead.com/npc=14371 sells drinks and a tailoring recipe (https://classic.wowhead.com/item=18487)

Duskwood

• [N] Kzixx sells a bunch of greens alongside a couple of recipes (Recipe: Holy Protection Potion and Schematic: Goblin Jumper Cables )

Eastern Plaguelands

• [N] Augustus the Touched is very particular: if you complete a quest for him https://classic.wowhead.com/item=15902/ will become available for sellings, with a randome green in it

Elwynn Forest

• [A] Antonio Perelli patrol through Lakeshire in Redridge Mountains and sells a lot of greenies

Felwood

• [N] Meilosh sells Formula: Enchant Weapon - Agility with enough Timbermaw rep which is a really nice enchant for rogues

Hillsbrad Foothills

• [N] Zixil sells a bunch of greens alongside a couple of recipes (Pattern: Earthen Leather Shoulders, Pattern: Red Woolen Bag and Schematic: Goblin Jumper Cables)
• [N] https://classic.wowhead.com/npc=3536/ sells a bunch of leather armors and some greenies (https://classic.wowhead.com/item=4794, https://classic.wowhead.com/item=4795 and https://classic.wowhead.com/item=4796)
• [H] George Candarte sells leatherworking supplies and a specific recipe, Pattern: Green Leather Armor. The item obtained from it can make you quite a lot of money, because it's an ingredient for a blue blacksmithing armor, Green Iron Hauberk

Loch Modan

• [A] Kat Sampson sells some leather amor and some greenies (Agile Boots and https://classic.wowhead.com/item=4789)

Redridge Mountains

• [A] Clyde Ranthal sells leatherworking supplies and a specific recipe, https://classic.wowhead.com/item=7289/

Silithus

• [N] https://classic.wowhead.com/npc=15293/ sells a ton of leatherworking reipes, provided you have enough Cenarion Circle reputation

Stonetalon Mountains

• [N] https://classic.wowhead.com/npc=4086/ sells many greenies and a specific recipe, Schematic: Goblin Jumper Cables
• [N] https://classic.wowhead.com/npc=4085/ sells many greenies

Stranglethorn Vales

• [N] https://classic.wowhead.com/npc=2672/ sells tailoring supplies and a specific recipe, https://classic.wowhead.com/item=10318/

Silverpine Forest

• [H] https://classic.wowhead.com/npc=3552 sells some leather amor and some greenies (Agile Boots and https://classic.wowhead.com/item=4789)

Stranglethorn Vale

• [N] https://classic.wowhead.com/npc=2843 sells many weapons, a specific recipe (https://classic.wowhead.com/item=12162) and a green dagger (https://classic.wowhead.com/item=12248)
• [H] https://classic.wowhead.com/npc=1146 sells many weaponds, a specific recipe (https://classic.wowhead.com/item=12164) and some greenies (https://classic.wowhead.com/item=12248 and https://classic.wowhead.com/item=12250)

Swamp of Sorrows

• [N] https://classic.wowhead.com/npc=11874/ sells leatherworking supplies, some greenies and a specific dragonscale recipe, https://classic.wowhead.com/item=15726/

The Barrens

• [N] https://classic.wowhead.com/npc=3683/ sells many greenies and a specific recipe, Pattern: Red Woolen Bag
• [N] https://classic.wowhead.com/npc=3684 sells some leather amor and some greenies (Agile Boots and https://classic.wowhead.com/item=4789)
• [H] https://classic.wowhead.com/npc=3682/ sells many greenies

Westfall

• [N] https://classic.wowhead.com/npc=1669/ sells some greenies (Agile Boots and https://classic.wowhead.com/item=4789) and a specific recipe, https://classic.wowhead.com/item=5640/

Wetlands

• [N] https://classic.wowhead.com/npc=8305/ sells herbalism supplies and sometimes has some https://classic.wowhead.com/item=929/ in stock
• [A] https://classic.wowhead.com/npc=2679/ sells a lot of greenies and a bunch of recipes: https://classic.wowhead.com/item=7114/, https://classic.wowhead.com/item=7290/ and Pattern: Green Leather Armor. The item obtained from the last one can make you quite a lot of money, because it's an ingredient for a blue blacksmithing armor, Green Iron Hauberk

Winterspring

• [N] https://classic.wowhead.com/npc=16015/ sells some reagents and https://classic.wowhead.com/item=21939/, needed for a single quest that will not be probably available until a latter Classic Wow phase

Felwood - Meilosh sells Formula: Enchant Weapon - Agility with enough Timbermaw rep which is a really nice enchant for rogues

https://www.wowhead.com/npc=844/antonio-perelli bracers :D

Alliance NPCs
https://classic.wowhead.com/npc=1669/defias-profiteer & Kat Sampson

Horde NPC
https://classic.wowhead.com/npc=3552/alexandre-lefevre

Neutral NPC
https://classic.wowhead.com/npc=3684/pizznukle

Sells: https://classic.wowhead.com/item=4789/stable-boots & Agile Boots

It might not be as rare, but it's definitely always on cooldown.
I'm pretty sure each NPC only sells 1 of them at a time.
At least on Alliance it is.

One of my favorite levelling weapons, and one that sells on the AH when newbies don't know its vendor-purchasable: https://classic.wowhead.com/item=12248/daring-dirk

Both factions can buy this dagger in limited quanitity in Booty Bay from: https://classic.wowhead.com/npc=2843/jutak

Less well known is that Hordies have a second vendor in Grom'gol, in: https://classic.wowhead.com/npc=1146/vharr

You can generally flip these on the AH for a couple of gold profit if you've got the bankroll by the time you get to STV.

Don't forget about the Shen'dralar Provisioner!

Once you gather enough information, can you maybe make the guide in the following fashion?

It would be great to see everything per zone and per profession. You can quote this post to get the code!
Thanks!

Ok, will probably convert this thread in a guide as soon as I have a bit of time

https://classic.wowhead.com/npc=3536/kris-legace is a neutral vendor who is hiding at the back of durnholde keep. real good for vendoring junk if youve been out questing for a while. also sells these greens: https://classic.wowhead.com/item=4794, https://classic.wowhead.com/item=4795, https://classic.wowhead.com/item=4796