They were designed for the mark series amps so yes it would be perfectly suitable for your mark III.
For $500 I would recommend that you ensure you are getting a cab that is in good cosmetic condition. If it's good cosmetic condition I would then look to see what speakers are loaded in the cab. If it has 2 C90's and 2 EV's or 4 C90's I would say it's a great deal. If it has 4 Vintage 50's then you should play through the cab to make sure it is right for you. The Vintage 50's are a little too "British" sounding for my tastes. If it has 4 vintage 50's i would not pay $500 for the cab.
Greg