The AH is now offline, so there's no way to check what crafting materials go for anymore. New ones are also Account bound, so you can't even trade them even if you wanted to.
It depends on what crafting material you are looking for. If you're looking for Subtle Essence and Fallen Tooth, your best bet is to farm as much gold as you can, and buy it from the auction house.
Currently, the crafting materials are selling for 20 and 45 gold, respectively. That is drastically cheaper than buying shop items, or even farming them yourself.
Source: this answer.
So, the answer fluctuates. In the event that the crafting material cost on the auction house rises above the cost of buying an equivalent magical item you can salvage, buy it from shops. In either case, gold find helps you.
For magic find, it will also depend on how long it takes you to find and gather magical items. The time investment will fall somewhere between the other two scenarios. It will take less time to find items you can salvage than it would to gather 1000 gold, but quite a bit longer than to gather just 25 or 100 gold.
The only time you want to really find magic items is when crafting material cost on the auction house goes above the cost of shop items. Which, at that point, becomes a stable economy for you on it's own. Buy magic items from shops, salvage, post crafting materials on auction house. Repeat. Use extra gold to buy items to salvage materials for yourself.
So, short answer, from least to most time investment, using Subtle Essence as an example:
- If: Crafting materials are dirt cheap (Costs less than you would get from selling magical items): Sell magic items and buy from auction house.
- If: Crafting materials are more expensive (more than selling your magic items) and less expensive than buying items from shops for salvage:
- If: Gathering requisite gold will take longer than finding magical items: Find magical items.
- Else Gather gold and buy from auction house.
- If: Crafting materials are more expense than buying magical items from shops: Buy items from shops, salvage materials, post on auction house for profit until price drops below viability.
Credit to LessPop_MoreFizz for the original information and inspiration.
Best Answer
Rares should be salvaged, always. They're completely useless once you fill every slot with a legendary item. You'll need the materials you get form scrapping rares (and also white/blues) in almost every recipe.
For the majority of the legendaries you find, their best use is to either extract their power using kanai's cube, or break it down into a forgotten soul. The one caveat is that it worth it to save any ancient gear you find just in case it comes in handy down the line. Ancient legendaries are rare enough that they're worth keeping.
For extra set pieces, there are ways you can use the kanai's cube to reroll it into another piece in the set. (10 souls, 10 dbs). This allows you to complete sets much faster.
Finally, there's almost no way to transfer useful gear between classes. Unless the item rolls near perfectly and you can reroll the main stat (say from str to int), or you can reroll it so it doesn't have a main stat, it won't be as useful as any legendary found on the wizard.
Even between same main stat classes, they typically use drastically different gear you won't be switching much other than a ring or two.