TIP #4: CUT YOUR LOSSES
Make a list of practical reasons why you both deserve and/or need the ring back. This is easy if the ring belonged to your grandmother; only a really terrible slag would fight over giving back something so personal and priceless. (Of course, you may be ending the engagement precisely because you’ve realized you were about to marry a terrible slag. Your bad!)
But if you want it back for financial reasons — and who wouldn’t, considering how much rings cost — be prepared to say as much. From a legal point of view (not that you’re planning on going to court over this) an engagement ring is something given as a promise of a future marriage. If that marriage never happens, the promise is broken; it generally doesn’t matter by whom or why in the eyes of the law.
However, if you’re the one who ended the relationship and you want the ring back so you can sell it, you might consider positioning the ring as a symbol of your relationship, thus owned by both of you, and offer to split the value. It’s better than nothing — and it could resolve the issue quickly and easily.