Good Question...
The wheel hub bore site info I know of, shows the X6 wheels having that front wheel identical to E70, with the rear X6 wheel being same as E53.
The Wheel Pics/info site I use, shows the wheel 'looking' identical, but that site does not contain wheel bore info.
X5 E70
10x20vorne
275/40R20 
ET 4020,5011x20hinten
315/35R20 ET 37
X6 E71
10x20vorne
275/40R20 ET 4020,5011x20hinten
315/35R20 ET 37
As to that set of wheels, there are a few possibilities:
the seller is mistaken, the seller is selling non-oe wheels, the set is legit but the rear wheel has been bored to 74mm, or any combo of above. I realize this is little comfort.
No idea how we/you can determine other than asking seller to carefully measure both F & R bores on the set that's being offered.
GL, mD