When you create a new private key it appears in the browser url, that part of the link after # is your private key.
I am lost WIF
Try to find the right link in your browser history. You can add page with wallet to bookmarks to save it.
What to do if the bitcoins do not appear on the balance?
Perhaps your bitcoin client is sending a more advanced type of transaction (SegWit, Multisign, etc). Create a Blockhain wallet and transfer the BTC to it first, then perform the transaction from it to the doublespend.online
I want to change transactions fee, how to do that?
You can click on Settings and change the TX fee.
Will the first transaction be confirmed?
When you click on send second, the first transaction becomes unacceptable to the miners. When the second transaction receives network confirmation, the first transaction disappears from blockhain.
How quickly should I send the second transaction?
You must send the second transaction before the first one receives the confirmation. The sooner you click on Send Second, the greater the chance of success.
Is this site legal ?
This site uses features that are found in almost all bitcoin clients. It is not safe to accept bitcoin payments with zero confirmations. We are against using the site for illegal purposes.
I own the site and accept bitcoin payments, how do I protect it from doublespend
You need to wait for at least one confirmation before starting to process your order. Having six bitcoin network confirmations is considered safe.