Long-time Internet Writing Workshop member Peggy Vincent, author of Baby Catcher: Chronicles of a Modern Midwife, recently discovered this article by Moira Allen on the Writing-World.com website.
When selling the reprint itself, you will generally offer nonexclusive "second" rights or "one-time" rights. "Second" serial rights can be sold over and over; the term simply indicates that the publication is no longer "first." (In other words, you don't have to worry about keeping track and offering "43rd serial rights.") Most reprints are sold "nonexclusively," which means that you can sell the same reprint to more than one publication, often at the same time. Sending out "simultaneous submissions" of reprints is also a little more acceptable than sending out simsubs of an original article; just be sure that the reprint isn't going out to competing markets.