Seen ads in olx and quikr about iphone 5s for 24000?

Wen i called them, they told that if i make payment they will courier it.
Is that really trusty?

Who knows. People can sometimes be trusted. Sometimes not.
Traders usually submit their waybills/lorry receipts/railway receipts to their bankers who would collect money from the purchasers of those goods and give away the lorry/railway receipts. Even then there's a risk of duplicate/fake material in the packages. You can ask for Cash On Delivery. Even then the goods IN THE PACKAGES they supplied may or may not be the goods you believed that they offered.
In case of a dispute, a court case might drag on for a very long time.
I WOULD NOT, PERSONALLY, DEAL IN SUCH TRANSACTIONS.