Ensuring transaction validity involves checking that they meet the blockchain's predefined rules and criteria, including:

1. Correct formatting: Verifying that transactions are structured correctly, with proper syntax and encoding.

2. Valid inputs and outputs: Ensuring that transaction inputs (e.g., sender, amount) and outputs (e.g., recipient, amount) are valid and consistent.

3. Sufficient funds: Checking that the sender has sufficient funds or assets to perform the transaction.

4. Valid signatures: Verifying that transactions are signed with the appropriate private key, ensuring ownership and authorization.

5. Compliance with blockchain protocols: Ensuring that transactions adhere to the blockchain's consensus rules, such as block size limits or transaction fees.

6. No double-spends: Verifying that transactions are not attempting to spend the same funds twice.

7. No fraudulent activities: Identifying and rejecting transactions with malicious intent, such as phishing or Ponzi schemes.

8. Compliance with smart contract rules (if applicable): Ensuring that transactions interact correctly with smart contracts, if present.

By validating transactions, nodes and validators ensure that only legitimate and authorized transactions are added to the blockchain, maintaining its integrity and trustworthiness.