T-Mobile supports any unlocked GSM phone using the 1900 frequency band,
so a Moto Razr should work just fine as long as the unlock ad claims ATT
or T-Mobile support. I'm not sure why they're omitting T-Mobile support
in the eBay ads. Phones meant for the Verizon network will not work
since they are not GSM phones.

If the phone is truly unlocked then it must work for voice phone calls
and text messaging using any compatible SIM as long as the radio frequency
bands match those used by the carrier. The AT&T and Cingular tags you see
may refer to the fact that these phones were originally sold and branded
by those carriers.

They will display the AT&T or Cingular logo when powered up, their menus
may contain special entries (like instant messaging, carrier-specific
ringtone downloads, etc) that do not work on other carriers, and their web
browser profile is set up for those carriers.

The web (WAP) browser issue is something else, as some carriers hide or
disable the option to change the data profile, effectively preventing you
from using the web browser when operating on another carrier. If this is
an issue to you, it is important to check with eBay sellers to be certain
that these menus are accessible and modifiable.

I see you've had your questions answered by other posts but what I'd
recommend if you're intent on buying a handset is to get one that hasn't
been locked to any network and has all the menu etc. options available.  You
won't have to enjoy/waste <g> time putting the object back to match the
manufacturer's manual then.

The original Razr is quite cheap if you can find one, assuming that's what
you're after.