I'm compelled by the numerous explanations I've had to give to numerous prospective customers in my line of work about used iDevices unlocking. Buying an iDevice (iPhone or iPad) is a kind of investment, so to speak, because you are buying superior quality hardware which, if utilised well will serve you in the tasks you choose to perform with it.

That said, let me explain what iPhone and/or iPad unlocking is all about, for the benefit of those who don't know.
iPhones and iPads come in basically two network configurations: Apple Unlocked and Network Provider Locked. What this means is that there are some that come fully untied to any network, sold directly by Apple. These are more expensive and can range from $600 and above. Then there are the Network Provider Subsidised ones that are locked to that particular Network Provider. They are mostly sold on contract by the networks themselves. These are cheaper because of the service contracts attached to them initially, but in the long run one might end up paying more than the cost of the Apple unlocked version in hidden network fees or early termination fees.

Buying a used iPhone (we will use iPhones as case studies in this narrative) therefore entails being certain you can unlock the device from the network it is tied to after purchase in order to be able to use it. Common sense comes into effect here. When I bought my iPhone 4S I was nearly taken in. I was offered one for -N-30,000. It was network locked. I asked the seller which network it was locked to. He said it was a Swiss network. I had my Galaxy S3 with me so I just went online and checked the cost of unlocking all Swiss networks. The costliest was around -N-8,000 or thereabout. I was happy; getting an iPhone 4S 16GB for less than -N-40,000 total! A mouth watering deal indeed. I had almost paid but then I asked myself, since I was online there and then why not just check the IMEI. I did just that and the whole deal fell through. First, I discovered that the iPhone was locked to T-Mobile UK (which would cost nothing less than -N-25,000 to unlock, if it was even possible)and it was barred, meaning that even if I could unlock it the cost would be astronomical because of the barred IMEI status. I backed out. When the seller saw I had seen through his game he lowered the price to -N-25,000 for the iPhone 4S. I just walked away. Some folks I know have jumped at this type of deal and are now left with very expensive iPods. I later got an already factory unlocked iPhone 4S for -N-50,000 somewhere else. Being factory unlocked and IMEI not barred, I have a full working iPhone 4S without any issues.

Back to the main point of our narrative, before buying a used iPhone ask the seller for the lock status and the network it is locked to. If possible, especially when it cannot be proven which network it is locked to, ask for the IMEI in order to check for yourself. They may not agree to that, but you can use common sense and a bit of tech savvy to get the IMEI without the seller knowing; note that it may be illegal collecting a phone's IMEI without the consent of the owner, but you are trying to protect yourself from being burnt in the deal. Check the IMEI online yourself or patronise someone you know can do it for you, maybe for a small fee. When you are satisfied that the iPhone can be unlocked at a reasonable price added to the selling price, then you can go ahead and make the purchase. Do not rush into it. Do not be afraid that you may lose the phone if you delay; there are lots of used iPhones in the market.

Another trend is where some people sell CDMA iPhones as iPhone 4S. They are not the same. Let me explain with this:

iPhone 5
Model number on the back cover:
A1428 - iPhone 5 (GSM model)
A1429 - iPhone 5 (GSM and CDMA model)
A1442 - iPhone 5 (CDMA model China - Only Sold In China)
There is a SIM tray on the right side that holds a "fourth form factor" (4FF) or "nano-SIM" card.
The IMEI is etched on the back housing.

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iPhone 4S
Model number on the back cover:
A1387 - iPhone 4S
A1431 - iPhone 4S (GSM China - Only Sold In China)
Front and back: flat and made of glass.
Stainless steel band around the edges.
The volume up and down buttons are marked with a "+" and "-" sign.
There is a SIM tray on the right side that holds a "third form factor" (3FF) micro-SIM card.

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iPhone 4
Model number on the back cover:
A1349 - iPhone 4 (CDMA model)
A1332 - iPhone 4 (GSM model)
Front and back: flat and made of glass.
Stainless steel band around the edges.
The volume up and down buttons are marked with a "+" and "-" sign.
There is a SIM tray on the right side that holds a "third form factor" (3FF) micro-SIM card.
THE CDMA MODEL HAS NO SIM TRAY.

Check the model number when in doubt.

CDMA iPhones are programmed for CDMA networks only and DO NOT have a SIM tray by the side.

The iPhone 4S is a dual mode "World Phone"; it has GSM capability and CDMA capability as well. This means it can be programmed for a CDMA network, or a SIM can be inserted and it will default to GSM. It cannot do both at the same time.

However, here in Nigeria, only the iPhone 4 CDMA can be programmed for a CDMA network. The iPhone 4S cannot be programmed for any CDMA network here in Nigeria yet; it can only be used on GSM networks here.

Folks have been coming to me saying they want to program their iPhone 4S for CDMA. I always have a hard time explaining to them that what they have is a CDMA iPhone 4. In most cases they bought them as iPhone 4S, and at the cost of the iPhone 4S. Tough world...

In rounding up, what we are saying is be vigilant, ask questions, do research, and do not be in a hurry to part with your money without being sure of what you are buying.

Nice

Thanks @Op, how do I confirm the phone status online? I have the IMIE with me.