How to Secure a Telegram Account:

Ask the Seller to Change the Phone Number
You won't be able to change the phone number for the first 24 hours on a new login, so have the seller update it to yours.

Have the Seller Manually Log Out of All Sessions
You won’t have the ability to log out the previous owner for 24 hours.

PC: Go to Settings > Privacy and Security > Show All Sessions.
Mobile: Go to Settings > Devices.

Ensure no active sessions are remaining except yours.

Enable 2FA
Activate two-factor authentication for added security.

Disconnect All Linked Websites
Mobile: Go to Settings > Privacy and Security > Active Websites and remove any active website connections.
Additional Tips

Customize your privacy settings for "Last seen online," "Forwarded messages," and group/channel additions.

It's highly recommended to enable the 2FA password for improved security.

✈️ @Guide