The following definitions are found on eBay itself.
See link ---->
http://http://www.ebay.com/gds/Baseb...2324072/g.html
"RARE CARD" - Expression used for one of a kind cards that are impossible to find in any condition and have a non existent presence in our hobby in relation to the cards that you see frequently. Years may pass before seeing one.
SCARCE - A card or series that are difficult to find but not impossible. These cards do appear but not frequently nor in large quantities.