No such thing as silly questions….
Yes, you could buy using one broker and sell using a different broker if you really wanted to.
Under normal circumstances that’s not something that someone would do, changing brokers is something that people don’t normally do frequently without good reason, but it’s possible to do it certainly.
If CommSec ceased operating or worse still went broke then ultimately you own your shares and can sell them with another broker. You’d just have to open an account with some other broker, quote the details (HIN etc) to do so but it can be done certainly.