In most cases the sims have a very tight expiration date, so they will be useless in a few weeks, so they sell them for whatever they can. I replaced my phone recently for a quad band (which they unlocked for me so I can travel with it), and it came with a SIM with a number on it with airtime, so to me that sim was pretty worthless. I'm betting there are people who buy phones to resell and then sell the sims separatly.