So either the ballast has a switch on the back of it.. making it a switchable ballast. Or it's a digital Ballast.

Digital Ballasts allow you to put both MH and HPS bulbs of approximately the same wattage into the same ballast. Meaning if you have a 400watt digital ballast, you CAN put a 430watt SonAgro bulb in that 400 digital Ballast.

I'd confirm what the auction is offering if i were you..