What to look for when buying cell phone from 3rd party

A few months ago I bought a Motorola V330 from T-Mobile for my wife; I
intended to buy another for myself once I'd used up the accumulated
minutes on the CallPlus service. However, T-Mobile is no longer selling
this phone (in fact, their offering for pre-paid phones is much smaller
than it was a few months ago).

I understand that I can buy one from a 3rd party and activate it with
T-Mobile. I've done some cursory checks on e-Bay and it seems the
advantage of buying there is that, although there is no rebate (there
was a $50 rebate from T-Mobile on a $199 purchase price), the price sans
rebate is about the same as buying from T-Mobile with the rebate. So,
no hassle and wait for the rebate.

I'm not sure how to go about this, nor what I should look out for, etc.
I've seen mention here of getting an unlocked phone, a phone with a SIM
card, etc., but don't entirely understand the process.

I'd appreciate some advice.

advTHANKSance

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Is man one of God's blunders, or is God one of man's?

--F. Nietzsche
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Joe Negron from Sheepshead Bay, Brooklyn, NY, USA