I bought a secondhand iPhone 4 on Ebay and as soon as I got it, I inserted my Sim card. However, my iPhone could not read it. After putting it in and out, I decided to reset the whole iPhone. Now, it says that I have to insert a valid card . What to do ?

What does it say when you look at Settings=>General=>Carrier?

Sounds like you bought a locked iPhone. If so, the only way to unlock

it is to contact the wireless carrier to which the iPhone is locked to see

if they offer unlocking and if you qualify. The other possibility is that

your iPhone has been reported as stolen and blacklisted - in that case,

it will never work.

Return the iPhone for a refund. If the seller refuses, contact eBay for

resolution. But an iPhone from Apple or authorized reseller to prevent

such problems in the past.

Return it and get your money back. If the seller won't cooperate, file a claim with ebay.

The phone was hacked or jailbroken and is now probably permanently bricked.