Is he offering you a standard used Sky digibox or one he is claiming to have modified somehow to receive Sky freely?
If it is just a used (or new) Sky box that is pricy, whatever the model (unless it’s Sky+) so ask him what the model number is and report back for a more realistic price.
If it is indeed a box that he claims to receive Sky freely on then be very suspicious, there is no publicly known Sky hack, unless this is it, but be suspicious.
It could be a scam, or it could be something else (like an analogue cable hack that gets some Sky channels or a box that hacks some other encryption systems and gets the free Sky channels)
Ask him for some more details and to see it working.