Most likely the seller had a newer kickstart in his Amiga, and therefore the filesystem on the hdd isn't compatible with Kickstart 1.3. If that is the case, you either have to upgrade the Kickstart or just format the harddisk.
You should be able to boot from floppy when inserting your floppy and then resetting the computer. If it still boots from hdd first, the boot priority of the hdd has been changed from the default value. You will then have to boot your floppy manually by executing the startup-sequence of the floppy, when you have the CLI window.
