If you take one wheel off of the car you'll probably get all of your answers, especially if it's made by BBS. They stamp size, offset, and bolt pattern between the bolt holes on the hub mounting area, back of the spokes, or if necessary in between the spokes or in the center of the face, whichever area is best. You're looking for something similar to the pictures below. It should also tell you the model name and number of the wheel.
On this one, #2 is the offset. ET##
#5 is diameter and width, which is pretty obvious.
#3 is the model number.
Edit: If I had to guess, I'd think they're actually 17x7.5" and around a 40mm offset. I'm not familiar with that model, but it's a nice looking wheel. Good luck with the sale!