You’re stuck – and I’m guessing!! But – this might be worth what you’ve paid for it.
Have you checked the BIOS setup?
Have you verified that the PC has USB 2.0, and that the drive is also USB 2.0?
Does the boot floppy have USB drivers on it?
Are the two computers (the build and the target) of the same make and model?
Is it possible that the USB chipsets are different?
That’s all I can think of off-hand.