In the email you get after making an entry, there is a URL for the entry that ends in ysid=####. That number is the id.
If another user makes the entry, then I’m not going to get that e-mail.
Also, the entry I want to delete right now is the one I made right after disabling the javascript to check for e-mail address (we have a community garage sale this weekend and one person is entering all 80 garage sales for us, and the unique e-mail requirement prevented her from doing that).
Unfortunately you would have to go into your database and look at the wp_yardsale table. Using PHPMyAdmin, you could look at that table and delete the entry from there.
If you want to search on a specific email, you can query:
SELECT * FROM wp_yardsale where email = 'user@whereever.com'