Can I use a 30A backfeed breaker when I'm not using the generator?
That should be physically impossible, and the "outlet" should not be an outlet (it should be a boxed plug).

If it's physically possible and the outlet is an outlet, it's not a generator input unless you enjoy having your service terminated when the power company finds out about it. So remove any "generator" label, take apart the suicide cord, and use the outlet as an outlet.

It's quite possible to backfeed in a safe and correct manner with a listed manual interlock (the main OR the generator can be on, both can be off, but both cannot be on, by physical interlock preventing that) and a boxed-plug input to the backfeed breaker. You absolutely do not need a (generally overpriced) transfer switch.

This is a manual interlock for a Square-D QO panel - there are other styles for other panel types/brands.



Here's a typical correct generator input, fed from an extension cord with a receptacle on the end that won't (easily) electrocute you.