Virtually all phones, when bought from a mobile service provider with a contract, are sold at a subsidized price. As a result they have a service lock so you cannot use SIM cards from other providers. The disadvantage of this to the consumer is that they cannot, for instance, use prepaid SIMs in a foreign country and rather have to pay the higher international roaming rates of their service provider. Different providers have different policies on unlocking ranging from never unlocking to unlocking for free for customers who fit some definition of a good customer (time of service, average monthly bill, etc). In England I believe that all companies are required to provide the unlock code for a "reasonable" fee. A few years ago Vodafone UK provided codes to unlocked my Pay as You Go phone for 30 quid. Dont know what they charge now. You can also pay third parties to do it, which was what you were seeing for sale.