What is the Plunder Chest

The Plunder Chest is an item present in some form in Mario Party 2, 3, 5 (as the Lakitu Orb), DS (as the Snag Bag), Super (as the Fly Guy Ticket), Superstars, and Jamboree.

When used (except in 2 but we'll get to it later), it steals a random item from an opponent of your choice.
A player can hold up to 3 items, therefore at worst your odd of stealing a specific powerful item someone is holding is 1 in 3.

There are powerful items you are really happy to steal/don't want to get stolen, the most obvious being the Golden Pipe (or equivalent).
When used it teleports you to the current star (and getting more stars than your opponent is the win condition), so it will serve as our example target.

Another important thing to note is timing: generally you only get items from item spaces and shops, things that happen during/after rolling; and you can only use items before rolling.
So if you get an item you can't use it until everyone else got a turn. For our purposes this means if someone already has a Plunder Chest, they can steal the item you just got before you have a chance to use it.

Basic counterplay

So you have a Golden Pipe (or whatever valuable item) and an opponent gets a Plunder Chest, what options do you have to play around it?

Use your Golden Pipe immediately so they can't steal it on their turn.
Fill your inventory with cheap items so they only have a chance to steal your Golden Pipe. (This is much easier to do reactively in DS as in this game you can buy multiple items per shop.)
Have someone else be a juicer target (either they have a lower proportions of cheap items, or they're a bigger threat so denying their Golden Pipe is more valuable than deying yours).

So that part doesn't sound too bad, the Pluder Chest discourages holding on to valuable items for too long. But what if the items are aquired the other way around?

Proactive Plunder Chest (aka the problem)

Let's imagine the reverse situation: an opponent gets a Plunder Chest and no one on the board currently has any item worth stealing. You arrive at a shop, which has a Golden Pipe, what do you do?

That's where I'm talking about threat of activation. As the MTG Wiki puts it:
"When faced against an activated ability, the defender has to play as though the ability has already resolved, [...]
Hence, the controller of the ability gets the strategic benefit of the ability without ever paying the cost or ever having to commit."

So back to our shop question, here are your options:

Buy a Golden Pipe, get it stolen (unless your inventory also has cheap items to reduce the odds), thus you wasted money and a shop visit, and your opponent got the Golden Pipe.
Buy a cheap item not worth stealing, getting some value from your shop visit and kicking down the Plunder chest later down the line.
Buy a cheap item not worth stealing, with the intent of holding it so you can get a Golden Pipe next shop and maybe not have it be stolen.

Both situations suck and feel bad, but unless you already have a full inventory you'll most likely choose 2 or 3.
Even if a Double Dice helps you less to get to the star than a Golden Pipe, you are getting something out of it, and your opponent isn't getting a free Golden Pipe.

But the other players getting to the shop will be at the same situation, and thus will also probably get a cheap item. Result: there still isn't anything worth stealing for the Plunder Chest, so it doesn't get used this turn cycle.

And now we see the threat of activation in effect: the Plunder Chest wasn't used, but its mere presence made everyone else skip on a Golden Pipe because that was a better play for them.
And if you were going with plan 3 (trying to fill your inventory before getting the Golden Pipe), the Plunder Chest that wasn't used also cause you to hold on to items that may have been useful to use.

So until that Chest gets used for one reason or another, no one is going to buy powerful items (or at least will be very hesitant to do so). That's why I consider that item to be toxic.

Plunder Chest the Plunder Chest

Just as a little note but the Plunder Chest is an item, and thus can be stolen by another Plunder Chest. So if someone has a Plunder Chest and you buy one in an attempt to counter, they can just (attempt to) steal yours.
In that situation you only wasted money because unlike stealing a Golden Pipe (or other) the Plunder Chest is still here, so the situation doesn't get unblocked.
Unless they fail to steal your Plunder Chest, so now you're the one warping the item buying.

Mario Party 2

MP2 is the one game where it works differently:

Players can only hold 1 item instead of 3.
The Plunder Chest steals the item of a random player (instead of a random item of a chosen player).
You can use the stolen item the same turn you steal it.

In that game the threat of activation is even stronger because you can't even flood your inventory to protect a valuable item, you need your opponents to get cheap items so that if you get a valuable one it gets protected.

I haven't played much of 2 at all so I'm not gonna elaborate on that iteration. I just know from other people playing that it's ridiculously strong in that game for that threat of activation reason.

How I'd fix it

If we're keeping item stealing an option (I think so, want some pressure to encourage people to use their expensive items), I'd rather have it be a space on the board you can visit, like Boo or like that space board on LEGO Party does.

This still results in threat of activation if someone is approaching the item steal spot, but because they can't hold it indefinitely the threat gets resolved once they get past it.