Quick question about buying phones on eBay

Hey guys,

I'm not really an avid eBay user (or a phone-buyer in any case) so I have a question that maybe someone can answer. I found the HTC One S (used) on eBay for a fairly good price. It says that it is the T-Mobile variant.

My question is, how exactly will this phone function in terms of network compatibility? I assume its not unlocked, so will it only work on T-Mobile?

Also, if it does only work on T-Mobile, does it mean the seller ended their contract with them?

And finally, will I be able to use T-Mobile prepaid with this phone?

Thanks guys!