Hi, all.
Just thought I would post my experience, and perhaps let someone else avoid the grief. The odometer died on my 300sd, so I thought I'd pick one up cheap on ebay. I won this item:
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1,1&item=7967855365&sspagename=STRK%3AMEWN%3AIT
from a seller who goes by the ebay ID of "mbzace". The games began almost immediately. The guy quoted me shipping by USPS Priority Mail, so I paid him thru Paypal. Two weeks later, it still hadn't arrived, so I emailed, and find out he hasn't shipped it. Then, he says he shipped, but used regular mail, so no tracking. Finally, I get with Paypal and file a complaint for merchandise not received. They investigate, find in my favor, and tell the guy to give me a refund. He only has like 12 bucks in his Paypal account, so I'm out the rest of the money. In the meantime, the speedometer does show up. I inspect it for about two seconds and see the the pinion drive gear has several teeth missing, and the black gear it drives has a BUNCH of teeth missing in several places. I ship the speedo back to him, Priority Mail with delivery confirmation, so I can prove my actions. What I didn't notice until after the fact: The guy's feedback rating looks very good. Then you click on the "buyers" tab- which shows how many people he's actually sold to. I think there were two.

Long story short: If I had known how much grief this was going to be, I would have just paid the hundred bucks to have the speedo rebuilt. I am not implying this guy is a scam artist (I think that would be too much work), but rather an extremely inexperienced seller who can't bother to do a transaction correctly, inspect his parts before shipping, or make good on a refund if there's a problem.

I have bought other MB parts successfully on ebay, so I'd say the venue is good, just this particular seller might be wise to avoid.