First let me say that the vast majority of my submissions are poems and stories that have not been printed before, so it only comes up when I want to include a story or poem that has been previously published. If you sold the print rights or the electronic rights to someone else and they haven't been returned to you, then you don't have them to sell to me or any other publisher. Think of it this way, if you loan a book to a friend, you don't have it to loan to someone else until the first friend returns the book to you.
I'm not going to go into the ins and outs of copyright and the rights that you can sell. There are several good books on the subject out there. Three of them are:
Copyright Plain and Simple by Cheryl Besenjak published by The Career Press
The Copyright Permission and Libel Handbook by Lloyd J. Jassin and Steven G. Schechter published by John Wiley & Sons, Inc.
Literary Law Guide for Authors by Tonya Marie Evans and Susan Borden Evans published by FYOS Entertainment, LLC
Get them. Read them. Memorize them.
For each short story or poem that you send out on submission, keep track of what rights you have sold to whom and for how long.
If I have any more trouble with this, then I may have to rethink my stand on accepting reprints.