When buying a SmartPhone on ebay, is it important to get one marked unlocked?
Like a Blackberry or a treo?

What are the dangers of used blackberry phone? What does unlocked mean?

thanx in advnce

"Unlocked" means that the phone is not tied to any service provider. 
You can get a locked Smartphone, but you'll have to use it only with
the service provider that originally sold it.  You can use an unlocked
phone with any provider (provided the phone is compatible - for
instance, you can't use a GSM phone with a service provier that has a
CDMA network).  I would either find a locked phone for the provider
you had in mind, or just check to make sure that a particular unlocked
phone is compatible with your intended service provider.

how can i unlock a locked phone?

an unlocked phone can be used any network within its capabilities (GSM
phones can only connect to GSM networks; CDMA phones to CDMA
networks). however, unlocking the firmware would automatically void
your warranty. The method isn't really straight "unlocking". For a
smartphone, you load a new firmware image to the phone. But this
depends on the bootstrap file installed. Older smartphone units have
old bootstrap versions, but the mobile phone providers tend to update
the bootstrap to avoid the software unlocking issue.

your best bet if you want to really unlock a smartphone is ask around
the people at Modaco.com

I was able to unlock my smartphone a few months ago, but reverted back
to the original rom because of some incompatibilities with my existing
phone network. That's also one thing to consider if you really want to
unlock the smartphone.