What I mean is, I've heard it's possible to pretend to send bitcoins so it shows up on the blockchain as unconfirmed. How easy is to do this? What would it look like on the blockchain... just a transaction that never gets confirmed or what?
I'm just wondering if it means anything when I buy a bitcoin from someone and it "shows up" in my wallet immediately without confirmations. Is that any evidence that they actually sent real bitcoins or could those easily be faked?
Essentially you need to send the same bitcoins twice, once to the address you want to create a fake transaction to, and the other to another address of yours. The time between the transaction must be short and you must make the real transaction more desirable to the miners than the fake one, this is achieved by giving the real transaction a high transaction fee. But as said, the window of opportunity is small.
Take a look at;
https://en.bitcoin.it/wiki/Double-spendingEdit/
You could also use block withholding, if you had some computational grunt at your disposal. But the window of opportunity is still small.
Edit/ Edit/
Sending a transaction without a fee will work sometimes especially if it's a large number of bytes and/or low priority. The transaction will never be included in a block and will read as unconfirmed until the memory pool is flushed.
