The most often seen example of this scam is the substitution of the Charon freighter with a piece of Carbon.
The advertisement in local will appear such as:
"WTS Charon for 1.2b , i need isk now !"
Clicking on the link will show the contract which will contain a Pay section of 1.2 billion ISK but in the Get section will be a piece of carbon:
|Yes, I'd like a cheap deal on a Charon|
Along comes Mr (or Mrs/Miss) Mark... "oh a Charon for under market price, I've been wanting one of those for a while..."
What Happens Now
As you can see in the screenshot the Info by Issuer clearly states he wants to sell a Charon for 1.2b but looking at the details section there is no Charon only a piece of Carbon.
The price of a Charon at the moment is ~1.325b ISK so the contract advertisement looks like a good deal. The mark will be expecting to get a freighter and they'll see something beginning with C in the Get section so may just go ahead and clicks the Accept button.
Except they'll receive a piece of Carbon, worth 300 ISK instead and see 1.2b ISK in their wallet.
There is a variation of this, not often seen now due to the freighters being much higher in price, that swaps the Charon for a Onyx Heavy Interdictor and the Carbon for a piece of Onyx Ochre.
How Can You Protect Yourself
Rule #1 in Jita is: anything in Local is a scam to take your ISK.
Rule #2 in Jita is: See Rule #1.
A visual check, and a hold on pressing that Accept button, would easily confirm this as a scam. There is no complex UI to navigate or confuse you. The contract clearly states 1.2b will be paid for a piece of Carbon. Apart from the advertisement in local AND the Info by Issuer at the top of the contract nowhere is Charon mentioned.
Pass on the contract and add the person to your blocked list.