After order confirmation, you can no longer cancel the sale. When items are purchased on Hypeboost, an agreement is formed between you and the private buyer. At this point the buyer has already paid for the item and expects the transaction to continue.
If you fail to complete a sale (this is what we call a ‘flake’) you may be fined and receive a one-time warning. After your second flake, you risk your account being blocked indefinitely.