Ebay Item Number  370163192605
 
Something doesn't look right about this:
 
http://cgi.ebay.com/Alembic-Bass-Guitar-USA-Made-Beautiful-Must-See_W0QQitemZ370163192605QQcmdZViewItemQQptZGuitar?hash=item370163192605&_trksid=p3286.c0.m14&_trkparms=72%3A1205|66%3A2|65%3A12|39%3A1|240%3A1318|301%3A0|293%3A1|294%3A50

The number might still be in the controls cavity and the serial number shown (handwritten) doesn't appear in the stolen section of the Alembic site.
 
Maybe it's a good Idea for potential buyers to ask why the TR cover was replaced and for a picture of the number in the controls cavity ;-)

It's a PayPal auction, so you should have some protection if the s/n doesn't match what was posted in the ad.  It looks like the seller is a music store, so they probably don't know why the plate was swapped out at some point over the last 17 years.

It is not unheard of for truss rod cover plates to become scratched and tarnished; so the possibility exists that it was replaced for cosmetic reasons.  Still, you hate to see the serial number missing.