Unsolicited contact may be viewed as spamming.

I was looking through some eBay pages and found the following that may apply here

http://pages.ebay.com.au/help/policies/rfe-spam-ov.html

Now I know you are just wanting to the best by your potential customers who place your item on their watch list, but being able to contact watchers would be abused by many sellers if it were available.

It may also mean that sellers organise "off-eBay" sales which would undermine the viability of eBay should it start to lose profits through fee evasion.