We all know how many dodgy bikes turn up on Ebay. This would make it extra-easy for theft victims and the police to check auctions, and might encourage more people to record their serial numbers too.
I blogged about the issues with Ebay a while back; this won't solve the problem of bike theft overnight but it's a step in the right direction. At present, stolen property fences have a huge low-risk marketplace at their disposal which even goes out of their way to make them look respectable. It's time something was done.