Gumtree Australia: how to tell if a seller is trustworthy?

I found an ad for a near new iphone 4s 64g selling for $150. The description within the add seems reliable but i'm aware that $150 for a new iphone 4s 64g is ridiculously cheap so i'm wondering if there's any tips to tell if the seller is trustworthy? What types of questions to ask? And is there any insurance policy that protects buyers from scamming on gumtree?

Hope it helps you in some way

http://gumtree.force.com/Help/knowledgeSafe

Now do you really think some outfit is going to say "We're not trustworthy, or dependable. We sell junk, and just rip you off."?
Don't bother asking them; use other outside resources.

Have a go with these:

"Web of Trust" should be on every browser. It gives an icon next to a Google search page, with green, yellow, or red dots, for each URL. New, unknown sites have "?"; use @ your own risk.
Right clicking any link you encounter gives a pop-up window> "View WoT Scorecard" takes you to the WoT site's index of that sites feedback. Very handy for those with liberal browser security or rookie experience dealing with websites. Http://www.mywot.com/

"Zscaler Safe Shopping" is continually up-to-date, via the Zscaler cloud security service, on compromised and fake online stores. It warns users when they visit one of the suspect domains.
Firefox add-on; http://research.zscaler.com/2011/02/zscaler-safe-shopping-stay-protected.html