1) Don't post the same question in multiple places
2) Don't repost the question looking for a different answer that is more to your liking
3) Scamming and theft are a valid part of the game. I say this because it's part of the larger point: you are never guaranteed access to any of your items - it's on you to protect them.
As Xakic said, this is one of the perils of buying from the NPC Market. Often, the items there are in locations that you cannot access and will never be able to (because they often end up on the Market via someone's death, and people just lock the items away somewhere).
To answer the question "Ho can I take my item from another player" I would graciously suggest investing in binders, or stun cuffs, Getting some Arrest and execute privileges and Kill the player once you locate him. Other than that or him magically returning, you really don't have another option.