Sold my Nextel phone and now Nextel is telling the person that it is stolen! (help!)

I am having a big problem right now and not sure what to do.

Last August I purchased a new i580 from Ebay to use with my Nextel account. I put my sim card in from my old phone and everything worked fine. I recently sold the same phone on Ebay and after the person received it they told me that when they tried to activate it Nextel told them the phone was a stolen phone and was in their 'negative list'.

So I called Nextel to find out whats going on and they told me that they didnt show this phone registered to my account. I told them that when I bought it I put my sim card in and registered it on my account online (although I dont remember if I entered in anything other than the model number) and I've been using it for the past 4 months. I gave them the serial number to the phone and they pulled it up and said the phone was registered to someone else. I asked how was it that I was able to use it for the past 4 months and they said because I only put the sim card in it and didnt register it is why it worked.

Does this make sense? Basically what they are telling me is that anyone could steal someone elses Nextel phone and put in their own sim card and it will work. Isnt there more to making a cell phone work on your account other than putting your sim card in it?

Any insight would certainly be appreciated.

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› See More: Sold my Nextel phone and now Nextel is telling the person that it is stolen! (help!)

Yes, it makes perfectly sense. If you find a nextel phone, you just insert your sim and works fine.. why? cause the sim is the one thats activated not the phone. You bought a stolen/lost phone and didnt call nextel to make sure the SERIAL was not in the negative file, but why would someone want to call and add the SERIAL if the phone is working fine? main reason, INSURANCE purpose, because if you add the SERIAL to your acct, thats the phone that the insurance will cover.
When you are using a phone model that is not on your account FEW of the times its not able to text right, or use the web properly too.