Disentegrating Necklace
This particular necklace could be worth 1,000... If you get it to the market fast enough.
Description
The Disintegrating Necklace is exactly what it means, it's a necklace, and, after a period of time, it disintegrates, just to go back to the place where you picked it up. It's exactly in every way a gold and diamond necklace, but if you sell it for the 1,000 gold the real one is worth before it disintegrates, you just won yourself a fairly large profit.
History
The Merchant of Kintro wanted to have a little villainous fun with his customers, making a necklace that would disintegrate within a couple hours, to make a few coins. He would then blame the disintegration on the customer, and refuse to refund the money. He took his idea to a wizard, who liked this idea. He made the item, and then thought he would have some mischief fun himself. He duplicated the necklace and placed them all over the land, and they could be anywhere. On a shelf, on the ground, anywhere.
Properties
Once the necklace has contact with a lifeform, it has two hours until complete disintegration. Run it to the merchant as fast as you can!
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? Responses (6)

A good idea, but short. the backstory could probably use some work.

I like it! (and Welcome!) Contact with a lifeform though? What if a cockroach crawls over it? Maybe something slightly more insidious as an enchantment?

This looks like a fantasy translation of a scam that was in the news a few years back, in which the scammer would write out a check, coat it with a mild acidic solution, and then cash it at a local bank. Mild acids have been used for a long time to erase parts of documents, but in this case the check would be soaked in enough of the stuff that within a couple of hours it would turn to illegible paper mush (the hope being, I suppose, that what was left of the check couldn't be used as evidence).
The backstory doesn't do enough. As written, the nameless Merchant of Kintro would be sharing a bunk with a family of dungeon rats within days, shortly after the Baron's third son bought one of these necklaces for his mistress. Either the Merchant could have a motivation (enough to risk facing the Baron's justice), or the necklace could be a more common item...
... If as few as half a dozen of these necklaces, or other similar "ghost-goods", were found out to be in the city, the customs of the area might change. Perhaps a jeweler would present his goods to the customer for inspection, and then they would settle down and discuss the price over a leisurely three-hour lunch. If the jewelry disappears during the lunch, the jeweler goes to prison.
(Plot hook: a jeweler friend of the PCs has been arrested for this sort of fraud. He was framed by a servant, or a thief disguised as a servant, who stole the necklace during the three-hour wait.)

A good idea but it needs a better motivation behind it.

I think the price should depend on regions

Stuff like this would quickly generate countermeasures and guard intervention.
For example, jewellers would simply not buy from people without knowing them or having someone vouch for them.
Waiting period of >2 hours before paying for items.
etc.
It would work well a couple of times, after that the 2-hour limit would destroy its value until everyone forgot about it.