What Things Can Be Safely Deleted?

[Jalad Lantana.3027]

Example, weapons and armor can be deleted once the skin is unlocked.

Most trophies but not all (apparently some may be required to acquire Legendaries)

What else, that can't be sold or have any reason to keep them, can be deleted to free up inventory space?

 

[Danikat.8537]

It's better to salvage weapons and armour rather than delete them, because then you get crafting materials you can use or sell.

I'm not sure it would be possible or practical to list every item which can be deleted in a forum post (especially because it may depend on you and what you're doing) but the information is on the Wiki. If you're ever not sure what an item is or what it's for you can look it up there.

I have an invisible bag on each of my characters and when I get an item I'm not sure about I drop it in there, then when it's getting full or I have time I go through and sort them all out, looking up any I'm not sure about. I've only had a problem when it's something from a new release and the info isn't on the Wiki yet (in which case I either try to find out in-game or wait for someone to figure it out) and when there's a few options and I can't decide what to do. If it's the second one I leave it somewhere I'll keep seeing it until I make my mind up.

[Jalad Lantana.3027]

Your right, I forgot to add "or salvaged". 

 

[corwin.8356]

Bob. You can totally delete Bob. 

[amymist.8674]

In general, if you aren't sure, look up an item on the wiki. It will tell you what its for, and you can decide if you care or not. It will also tell you how the item is acquired, so you can figure out how hard it is to replace if you decide you want to do so.