The Salvage Yard is one of the most rewarding, if not the most rewarding, small buildings that you can have in your Garrison. It’s actually a small building that should occupy the 3rd small building plot in each Garrison. That’s because it does not complement any profession, and it can provide you with plenty of cool and / or expensive items, that you can sell in the Auction House, along with materials for all professions.
To build your Salvage Yard, you need to be at least level 96, and complete quite a chain quest, at Pinchwhistle Gearworks, in Spires of Arak. The quest chain begins with the quest “Defungination“, given by Kimzee Pinchwhistle (60,72), in Spires of Arak. The quest chain ends with the quest “Flame On!“, which you also turn in at Kimzee. You will receive as reward from this quest, the Garrison Blueprint: Salvage Yard, Level 1.
Now, to actually make some profits with this building, you need to raise it to level 3. To unlock level 3, you must complete the achievement Salvaging Pays Off, which requires you to open 100 pieces of salvage, obtained from follower missions. The achievement is account wide. Therefore, if you have a Salvage Yard on 2-3 characters, you can obtain and open more pieces of salvage, helping you complete this achievement faster.
Once your Salvage Yard reaches level 3, you will begin to get Big Crates of Salvage from follower missions. And by opening these boxes, at your Salvage Yard, you will get plenty of green items, which sometimes sell for a lot of gold as Transmogrification gear, or worst case scenario you can vendor them of disenchant them. Of course, you can get epic BoE gear, of various levels, including level 665 gear, which you can sell at the Auction House for 10-20k gold.