Yes, in eBay under "My eBay" go to "Searches You Follow" and enter the desired search string. Whenever an item comes up for auction that matches the search string, eBay sends a notification e-mail complete with a link to the auction.
When I am the person contacting the seller for additional information I always thank them for taking the time to reply. Here's the body of my thank you. I modify it to fit the situation.
Thanks for your prompt reply. I am not a buyer. However, your information has contributed in a small way to the search for Amelia Earhart. In 1940 a sextant box was found by the British on a small island in the Pacific along with human remains. The box is lost. The numbers on the box were reported. The thought is to match numbers on available boxes and sextants to see if a pattern exists that might link that box to Fred Noonan, Earhart's navigator. TIGHAR.org has visited the island several times searching for artifacts that might link that place to Earhart - with some success. I've posted your numbers to the TIGHAR forum where several hundred folks will be made aware of your auction. Good luck selling your piece of history.