I wanted a player trap that is undetectable, conveniently collects items and transports them hidden to your home, and requires no maintenance.

This is what came out:
Procedure:
I have also uploaded the process as a video on YouTube:

1. trigger:
First, the player walks down a hallway. Behind a corner lies an item on a pressure plate:

As you walk down the hallway, you collect the item through the wall, thus taking the item off the pressure plate, which triggers pistons that open the floor. When the item is removed from the pressure plate (by player or despawn), a dropper throws a new (cheap) item onto the pressure plate and the pistons close again.
2nd trap:
When the bottom opens up, the player falls into a deep hole:

This must be so deep that the player dies in any case - even with armor and "featherfall" enchantment.
3. collect items:
At the bottom, funnels collect the items that the player has lost while dying and transport them into a minecart. As soon as all items are transferred, the minecart starts automatically. How this works, I have already once here uploaded.

4. remove and hide items:
The player is respawned in the meantime and will try to get his items back. To do this, he will dig himself into the trap and follow the tracks of the Minecar to see where it has gone. Therefore I still have the Minecart secret passage built in, so that the player does not find the actual path of the tracks, but runs the wrong route.

5. store items
The Minecart drove to your home and unloaded the items with the unloading station linked above.

When you are sure that the victim has given up looking for the items, you can send the minecart back.

Download:
The map can be here download.

Still existing problems:

• The player sensor is unfortunately not completely maintenance-free, because it slowly consumes items and you have to refill it from time to time. In addition, the player usually notices that he has collected an item there, and can thus find the sensor.
• In search of his items, the player is likely to dig up the Fallle. You'll have to fix that, too. To make the digging up more annoying, you can build the most risky places out of obsidian.
• The Minecart loading station unfortunately doesn't work properly if the player has more items than will fit in Minecart or the items don't all fall into the hoppers at the same time.
• The secret passage is open all the time, as long as the minecart is on the side of the trap. Of course you could change that with a detector rail, but then of course the player would find it and would only have to follow the redstone line underneath.

If anyone has a suggestion for improvement against these problems, feel free to post it in the comments.

Instructions:

Tips for the construction:
It's best to build the trap somewhere where enemy players will pass by from time to time. In front of the entrance of the trap it is best to write "Do not enter!", then it is the player's own fault if he goes in there anyway.
The Minecart track should be a bit longer and contain a few curves so that the player doesn't find the secret passage. You can choose where the fake end of the track leads to. E.g. into a subterranean lava lake, then the player thinks the minecart with its items has sunk into it. Or you just let it end somewhere.