the "108" refers to the chassis number/series to which the wheels would be mounted. The 300SEL 6.3 was a [W] 109 chassis and hence some of the confusion.
That is not to say that there are not a great number of W108 parts on a W109, as the 109 was a stretched, or long wheel base version of the W108.
There is nothing wrong with trying to get as much information as possible before selling the wheels. Not every potential purchaser is as straight up as "hilld" was in his request for right of first refusal.
You also might wish to sign-up and post here:
M-100 Group Classifieds
This one additional place where "6.3" owners hang out on-line...
Good luck,
JR