A GW2 "bag-opener" is a character which you keep at a specific level, and use to open loot bags.

This works because many loot bags drop what's called "leveled loot." This means that the loot you get from the bags is the same level as the character which opens them.

Salvage the leveled equipment and armor into crafting materials, then sell the materials on the Trading Post.

This is also a great way for Free-To-Play accounts to make gold in GW2. Free accounts get two character slots, and are allowed to sell unrefined crafting materials on the Trading Post.

How do people make so much money in the Silverwastes?

Most of them use bag-openers. Here's how it works:

What level should your bag-opener be?

For the most current recommendation, check http://silverwastes.loltools.net. The standard recommendation is to use a level 49 character. This will get you Tier 3 and 4 materials, which are usually the most expensive.

You can also use bag-openers to "farm" a specific tier of wood, cloth, and leather.

Here are the materials tiers, along with the character level you want to use to get them:

Tier 1: Green Wood, Jute, Rawhide: Level 1-20
Tier 2: Soft Wood, Wool, Thin Leather: Level 16-33
Tier 3: Seasoned Wood, Cotton, Coarse Leather: Level 31-50
Tier 4: Hard Wood, Linen, Rugged Leather: Level 44-63
Tier 5: Elder Wood, Silk, Gossamer, Thick Leather, Ancient Wood (rare), Hardened Leather (rare): Level 59-80