If you're building your manifest, you're going to want to build those armours that unlock new characters, as well as improve on the armour value of your existing characters. Here are just a few tips on comparing items, and how to salvage effectively while building up your inventory.
Things to avoid salvaging (mostly)
Armour Upgrades
You can't craft these in MP, so it's not a bad idea to hang on to them, unless you're quite certain that you won't need them.
Accessories
These are pretty rare, so it's a good idea to hang on to the best one of any bonus you have. They also stack, so combining two of the same bonus can be helpful, as well (I will update this upon confirmation of whether two identical rings stack with each other.)
Your best weapon of any type
This doesn't just mean single-handed, or two-handed, but also the sub-category, such as "maul," "sword," or "Axe." Weekend Challenges can be lucrative when building a manifest, so keeping your best of any weapon type on hand is a good idea, just in case. We don't yet know what form these will take going forward, so I even hang on to my best Staff of each element type.
Two categories you can pretty safely just keep your best in are shields and bows. There aren't any sub-categories of those, so it's unlikely you would need more than one or a few of your best.
This you Can (almost always) Safely Salvage
Low level common weapons
You'll replace these at the drop of a hat.
Weapons with the berzerk attribute
These don't tend to work out so well for most players (hi Dragonracer.) They return a high damage penalty against the character using them.
Comparing Weapons
Don't just rely on a single stat, such as DPS, when comparing weapons. Try to take the whole weapon into account.
While one weapon may have a higher DPS than another, the second weapon may offer highly beneficial bonuses, especially if it is Rare or Unique. A high critical damage bonus on a weapon being used by an Assassin with guaranteed criticals from Stealth could benefit them more than extra raw damage. Heal on kill, having a percentage chance of creating an extra effect on hit or kill, these are abilities that can elevate a weapon's value, and you should take them into account when deciding what to keep, what to upgrade, and what to use.
Crafting Armour
When first crafting a set of armour, the actual tier of the crafting material has no effect. Try to use your Tier 1 Common crafting materials when making an armour set, and save your higher-tier and rarer materials for when you are crafting upgrades.
I'll update this post with new information.