I have a hoarding problem too, so I will share a few tips.
As others have said, the wiki is really good in this game, and can be launched from the chat. If you’re wondering how useful something is, the wiki page will have a list of aquisition methods so you can verify if you can get another copy later. Also good for clearing out the turn in to npc items, as it tells you where to find them.
For things that aren’t account/soul bound, I usually just sell them on the trading post if they aren’t immediately useful. If you need it again, just buy one back for a slight premium. I think of it like a storage unit.
If you don’t want to do the above, look into creating a personal guild just for guild storage. Again, this only works for stuff that isn’t bound and takes some up front work and money, but in terms of cost per slot, it is very efficient.
Mule characters are a common if cumbersome solution to this. Just create a new character and offload stuff through your bank. Anything you need again later they can send back.
I would recommend upgrading to 18 slot bags when they seem affordable. Keep in mind you get most of your money back from selling your old bags on the trading post, so don’t worry about settling for something affordable now. There are also some achievements and thing that reward large bags.
Most vendors have a sell tab with a button to sell all junk. Use this, as there is no other use for those items. Items that mention “this has no value outside of a collection” or similar can also be safely sold to an npc, as they are added to the collection on aquisition. If they are leftovers for renown hearts or similar, just delete them.
If you have excess for non-ascended crafting components, I would go to the bank and sell off the stack and repeat the next time it gets full. Ascended crafting mats are unfortunately account bound but you can get rid of these with gobblers/converters for a reward if you want to get those.
If you haven’t already, get familiar with two buttons in your inventory: deposit all materials and compact. The former moves stuff to its dedicated slot in your bank and the latter gets rid of gaps in your bags so new stuff goes to the bottom. The dedicated slot is also accessed for crafting too…
Rare/yellow gear and below can generally be salvaged (or sold to the trading post) without worry, since they are common enough random drops. There might be some bound gear from your initial character creation, leveling, or achievements that are technically unique, but they are replaceable with much better things anyway. Any unique skins would be granted on salvage/aquisition. Again, you can use the wiki if you want to be sure.