Shell, I took my old engagement ring to jewellers and pawn shops and they didn't wanna know... it was £867 new and I know this because my ex was a waste of space so I bought it.
Anyway I listed it on Ebay and described it to the best of my ability and I got over £100 for it which I was really pleased about (and surprised given what the jewellers had said!)...
Depends how much you want for it really. Personally, I just got to the stage where I'd had it 6 years and wanted rid!