If I buy an iPhone 5s online, will it work for T-Mobile?

I wanted to buy an iPhone 5s or 5c from eBay or Amazon, or the Tmobile store online. If I buy it from ebay or amazon, how do I activate it? What kind do I have to buy? And also, how much do you think the price will be on Black Friday or when iPhone 6 comes out?

No matter where you buy it, it has to be a prior Tmobile or unlocked phone in order to work on Tmobile's GSM signal network. Verizon Sprint won't work. Unlocked is very high so try to find a Tmobile used if you prefer. 5S price will come down some during pre-order for the 6 in late Sept. On contract with Tmobile will probably be best for your wallet. You've waited this long, a few more weeks to get a decent deal won't hurt. 5C and 5S will be much cheaper when the 6 shows up. Best luck.

You activate it once the SIM is in it.
Either unlocked or locked to T-Mobile
No clue on price.