Is it worth paying 100 for an IPhone 4(or 4S) locked to O2?

Is it worth paying 100 for an IPhone 4(or 4S) locked to O2?

In the UK, £100 for an iPhone 4/4s locked as you describe would be a suspiciously low price even if the phone was cosmetically poor but fully functional and in need of no repairs.

If you know and trust the seller, then go for it. If you don't then tread very carefully as the phone may well either be stolen, hiding some intermittent faults caused by water damage, have a knackered battery which won't be obvious until after you've handed over the money, or you might find that after selling it the owner reports it lost or stolen to an insurance company in which case the phone gets a permanent network block.

If you're buying from a private seller who is not a close and trusted friend then ask to see photo I.D. Before you buy and get a signed and dated receipt with the seller's name and address printed on it, and arrange to pay by cheque or bank transfer. That way you have proof of sale and also traceable proof of payment if the seller fraudulently declares the phone lost or stolen afterwards. It still doesn't protect you from buying a phone with expensive and hidden faults though.

For about £60 more you can buy a grade C example from CEX. It will have significant cosmetic defects but will have been tested and certified fully functional and will have a one year guarantee, so if it gets blocked or has undetected major faults you can get a full or partial refund or a replacement.

Try to get one which has not had iOS 7 installed. Many users report that their iPhone 4 models have become laggy to the point that they're effectively unusable after upgrading from iOS 6 which was perfectly good on those phones. IPhone 4 is affected more than iPhone 4S due to the difference in processor speeds.