I just did the same thing.  I was able to get the phone to work with my  
T-Mobile SIM card.  However, my phone had problems with SIM cards in  
general and I had to take it to Cingular to send to Motorola for repairs.  
It would only work if I squeezed the phone where the SIM card was located  
when booting the phone.  This would generally get it working.  After some  
period of time, however, it would suddenly say Insert SIM.  Did this with  
both my T-Mobile SIM as well as my wife's Cingular SIM.  I suspect that  
your phone will work with T-Mobile.  If it doesn't, and the seller told  
you it was unlocked, I would return it or request that he get the phone  
unlocked.

If it's really unlocked, and not defective, it should work. If it's not
unlocked, you'll have to send it or bring it to a place that unlocks them;
there is some issue with unlocking it remotely. I know that there are places
in Las Vegas and Oakland, CA that will unlock the V400 for $30.