If there is an Apple user group or two in your vicinity, they may be contacted and if asked perhaps they know of someone among them who has the Classic OS 8.1 or 8.5, etc on Floppy Disk. They may be able to make a copy for reasonable cost of supplies, etc. Or someone among that kind of group or an independent Apple specialist may know of someone who can help you erase the hard disk drive and install a new system on it.
At least if you can erase, and restore a system on the computer, and/or get a copy of the boot system on a floppy disc to go with the PB 5300c, it could be a little more useful or appreciated as a vintage item.
Since some persons like to collect those older portables, it may have a residual value even if lacking the accessories and parts, discs, etc that made it a working kit when new. I was looking online at lowendmacs.com on their software page and there are a few listed or named companies that may still have older Mac OS on floppy. There are also some links to update a few older systems, still.
Anyway, without the correct SCSI cables or other adapters, or compatible way to attach and use another computer and a compatible utility, the cure to this issue revolves around finding the correct stuff.
You could remove the hard disk drive, if not terrible difficult. And whack it with a big hammer. Or if you find someone who wanted the old PowerBook 5300c, and had floppy discs of the OS, and extra parts, they could erase it.
From where I am, there isn't much available. Older repair shops have gone, and a small population base makes those who like good vintage stuff have to look ever so harder to save it from destruction.
Hopefully someone with a method within practical means will reply.
Good luck & happy computing! 🙂