I recently purchased a mobile phone through e bay, upon trying my SIM card i got the message "invalid sim" displayed.
I did a bit of research online and found out that the phone was probably reported lost/stolen and that i needed to contact my operator customer services.
I am on the Orange network and the phone i purchased was also locked to Orange.
I contact customer services and they asked me for the IMEI number, i passed this on to them and they informed me that the phone was blacklisted, they asked me where i had purchased it from and advised me to contact the seller and also e bay.
They advised me that if in the future i purchased a second hand phone to get the IMEI number first and have it checked out with them to ensure that the phone was not lost/stolen etc.
Fortunately on this occasion i got my money back (£180.00) from the seller without a problem, others may not be so fortunate and for the sake of a quick call to the mobile operator you could save a lot of money.