I have the 12 bolt for my car apart at the momment and I am trying to figure out why it is setup the way it is. It has a 3 series carrier (identified by the number on it) and a gear with a spacer. The gear has 41 teeth...the pinion has 11 teeth. The pinion gear has some markings on it, 8620 82-0630-1. Why was the spacer used? Is the pinion smaller for some reason, causing the ring gear to need a spacer?

Sounds to me like it is not a 3 series carrier. How about a picture

Here are some pics...

I know that some 4.10 gears will fit a 3 series with a spacer. But I've never heard of 3.73 with a spacer. Maybe B.G.H. will respond, he will know.

There are special 3.73 gears that are made to fit a 4 series differential. To use one of these on a 3 series would require a spacer. You might have one of these thin 3.73 gears.

If that's the case, would the 3.73 gear be worth anything...as far as resale value? Both gears look to be in very good condition.

Yes, there are many people with 4 series differentials that want to get away from the 4.11, 4.56 or 4.88 gears that they have now. The thin 3.73 is as far as they can go without changing to a 3 series differential.