This assumes that your password is never leaked, changed, or reset. What happens if Ebay's password database were exfiltrated? You now have the following proble

This assumes that your password is never leaked, changed, or reset. What happens if Ebay's password database were exfiltrated? You now have the following problems: 1) If your password is cracked, a hacker seeing it might deduce your algorithm and then deduce that your Amazon password would be 'myemail+mo', etc. Now your scheme is out the window. 2) Even assuming your password is salted and hashed securely such that it can't be decrypted, Ebay would probably reset it as a precaution. So now you've got to change this password. Maybe you decide to do the 3rd and 5th letters of the domain name instead. So now your scheme is 2nd-and-5th for all but Ebay, and you've got to remember that the Ebay one is unique. For now. As more leaks or resets occur down the line, however, you'll have more exceptions, to the point that you'll be forced to maintain a list of what KIND of passwords each site uses. In which case, why not just use a password manager?