Hmm possibly they don't have authority to give out this information however I think they would need to see the device to give a proper appraisal. Maybe have a look at Ebay or Gumtree to get a rough idea what they are worth.
That's surprising to hear natjen09 as the info we have is that it should be a fairly straight forward process to obtain this information in the system they have,
If you have a social media account, please send a PM to Optus on Facebook or a DM on Twitter with the details. Mention that you have already spoken with Live Chat who didn't help with this for some reason.