PvP: this player (situation 1) has already started it ...
... so your group, if you all agree, can finish it.
This answer comes from experience, but whether or not this works for your group depends on how the rest of the group feels about this character/person doing this magic item theft/hoarding thing.
Step 1: ask the rest of the players if they care.
If they are not as upset as you are, then stop reading here. You are done with this line of inquiry, and you need to decide for yourself if you want to raise this issue one-on-one with that player, in terms of their anti-team, anti-social behavior. For that, see the other answers.
Step 2: set up an intervention, Out-of-Character
If, on the other hand, the rest of the group are also upset, then you can set up an out-of-character intervention. With all players present, make your case that they are harming the fun of all other players: share or leave the group. You all have to agree that this is what you want, or it won't work. Then, proceed to the "DM" answer section and get into a "how do we share the loot" agreement for the group.
Two talking points to remember to bring up: (1) the game is supposed to be fun for all of the players, and (2) D&D is 5e built as a team game. The whole party had to defeat enemies and challenges to get the magical treasure; it is neither fair nor fun to have one person reap all of the in-game benefits.
See what the reaction is. (And see "Then What?" in this answer).
Step 2a: set up an intervention, In-Character
A possible sequel to step 2, or a stand alone course of action.
This approach takes a united front to pull off, and it can work, but it entails some risk.
With D&D 5e's bounded accuracy, it is pretty sure to succeed.
After the next encounter, as / when loot is being divided, in-character have one of the PC's instigate a "you are holding out on us, cough it up" dialogue with the offending PC.
If they refuse, begin combat and in-character subdue the PC by dropping them to 0 HP (using the knock out rule helps here). Then, loot their belongings. Take what you like and leave the rest to the sorcerer. Then, assets permitting, revive them with a healing potion or a healing spell.
What, you propose that we gang up on this one person?
Yes, because that is what an intervention is, in real life. A bunch of people who care for a person with a terrible habit - I have seen this done with both drug and alcohol habits IRL, and it often is not pretty - gang up on them and confront them with their problem.
It sometimes works, and it sometimes does not. No matter what happens there is some pain involved. If I am never involved in one again it will be too soon.
You all are dealing with a game - nothing of serious importance.
This character, and more importantly, this person, has already initiated a PvP style, anti-group attitude by taking and holding, and destroying, magic items that the group earned together. This is not a team player. As described, this is a toxic player.
Yes, approach (2a) has risks: dealing with real people often does
Yes, this approach recommends responding to PvP with PvP.
Yes, this approach may cause hurt feelings.
Yes, this player has already caused a problem for your group; a problem for you (else you would not have asked for a solution here) and perhaps for other players in the group.
- Note: if your number one priority is not to cause hurt feelings, then course of action 2a is not for you, and you will certainly not solve this problem. Even with course 2 you may end up with hurt feelings.
They either quit the game / leave the group, in which case you have gotten rid of a problem player, or, they learn a lesson and perhaps become a better team player thanks to your group letting the player know that their behavior is outside of this gorup's acceptable limits. Hard to say. People can respond to peer pressure in non-linear ways.
I have seen this approach work out in one of two ways.
In some cases, the person can't handle the chastisement and quits the group. And where we all knew each other, some friendships got harmed, but the tension had already been present due to this anti-social behavior. (arrgh, late teen / early adult egos clashing ...)
In others, they accepted that their peers sent a message that they had stepped out of line and they adjusted their behavior. Peer pressure can work in a positive way, sometimes.
I can't predict what their reaction will be, but it will most likely be one of the two that I mention.
You already have a toxic player.
If the other players don't mind that they are playing with a toxic player, you have to decide if you want to play with this group or not. You can walk, perhaps, which means that you need to find another game. But if they don't mind, and you want to keep playing with this group, then just deal with the fact that this PC is not to be trusted with loot and take such precautions, in-character, as needed. This can work out, but in my experience that is the exception not the rule.
There is no Easy Button
You have to decide how much of that kind of behavior that you are willing to put up with.
To quote an old, old sorceress: the choice, Willow, is yours.
For your DM role (situation 2): delegate loot division to your players.
It's their problem to work out. I have an answer for that here, in detail, on why you ought to delegate this to the players.
Here is a portion of that answer:
Have the players in the party establish a loot division method.
Loot division is best arrived at through an intra-party consensus. It's heavy handed for the DM to dictate to the players how loot will be divided among them. The DM has enough work to do already. The first step is an Out-of-Character step. If necessary, the players can apply any In-Character steps.
Before play starts for the next session, invite all of the players to get involved in a discussion for how the party divides up loot. Let them kick it around. Once they come to an agreement, have them formalize it, write it down. Notes on a card or a sheet of paper suffices. You can participate in the discussion, if you like, but don't dictate; address various options and their benefits as they come up.
Respect their decision, and as the neutral referee make a note of it. See how it plays out during the next session. If you want to offer a nudge to one of the other players, you can offer a passive Perception or Investigation check and tell the player something like "you noticed the Barbarian looting during the battle." See what kind of in-character discussion or interaction takes place. Let the PCs interact to resolve the conflict, if any arises.
If the rest of the players don't take action on such prompts, then there isn't a problem to solve, yet, since it isn't important enough (from their perspective) to take action. Let them apply peer pressure to inform any change if they aren't happy with the status quo.
Your role as DM is to act as neutral mediator to any such discussion, and to calm things down if any of this discussion gets heated.
You, as a DM, have enough to worry about.