"not a system disk" means the floppy you put in is not a boot disk. If you pop-in just any ol' floppy into the drive before booting, you'll get that message (unless your BIOS/MB is smart enough [new enough] to "think" around that).
Also, since the auction vendor obviously screwed you, think of it as a lesson to watch out for in the future. Second, if you have any details on this vendor, be sure to let people know. One good thing to do when running into crud like that is to tell others so they don't fall into the same trap.