according to the ebay "links policy" found at http://pages.ebay.com.au/help/policies/listing-links.html it's a grey area.
anything that directs a person away from an ebay page, whether or not it is a link - if it encourages the person to go to another page, then it's prohibited by the policy.
there are a couple of exceptions - but it's best to read the policy for yourself.
the member-to-member contact policy prohibits exchanging of social media links, contacts or usernames - so i would dare say that this probably means it's a no-go in a listing also.
that policy is viewable here - http://pages.ebay.com.au/help/policies/rfe-spam-ov.html
others probably have more experience in this area - but i think they're more likely to deny it than allow it.
