I saw this thread about someone wanting to sell their iphone 5 and someone saying it sounded a bit suss and someone even suggested it was stolen.
There's a quick way to find out if it is stolen. Though it only shows phones that were reported stolen/lost. If it's reported AFTER you bought the item, than it's bad luck, but you'll find the phone will end up being locked.
http://www.amta.org.au/pages/amta/Check.the.Status.of.your.H…
Also, to check to what provider an iphone is locked and to find out warranty status, use this site:
http://www.imei.info/check-warranty-unlock/
I'm sure a lot of OzB's already know the above mentioned sites, but if I can prevent even just 1 person from buying into a disaster, my work is done :o)