If your story has already been published -- and in most cases, posting your story online COUNTS AS PUBLISHED* -- don't send it to a market unless they state in their submission guidelines that they accept reprints. If they do, and especially if they *also* buy unpublished stories, mention in your cover letter that you are submitting it as a reprint, and where/when it was published.

(* AW's SYW is password-protected in order to preserve your first-pub rights.)