Selling a iPhone on eBay

I have a question on how can I sell my iPhone 4s now that the iPhone 5S is coming tomorrow.
I have bought things on ebay but never sold anything mine, but now I would like to do so with my iphone 4s.last year I bought an iphone 4s unlocked from ebay (was from at&t) I wanted to work in tmobile and it worked but now after a year I want to sell it for a iphone 5S. I went to imei.info for my iphone status and
Model: IPHONE 4S 16GB BLACK
Activated: Yes
Telephone Technical Support: Expired
Repairs and Service Coverage: Expired
Purchase Date: 15.02.2012
Contract Until: 15.02.2014
Blacklist Korea Networks:
Blacklist Australia Networks:
Blacklist Japan Networks:
Blacklist Telenor (Sweden):
simlock: unlocked
yes my phone is a unlocked one but what I don't understand is were it says activated: yes. Does that mean my iphone is locked to tmobile and I can't change the carrier? Or it means that my sim is there and its in use? How do I deactivate the iphone from tmobile and sell it and from there the new owner can use it on another carrier or may use it in the same network as I'm but in his tmo account?
do I simply remove my tmo sim card and format the iphone to factory settings and then sell it or I have to take to tmobile so they can know ill be selling it? I bought the iphone 4s from ebay 2 months later after the 5 was announced.according to imei.info it say purchase date was at feb 15 2012 but I bought it on November so I assumed that that was the date the original owner but the phone. I have no plans at all to move to another carrier (more than 2 years) so once I sell my iphone can I from there buy the 5s or they'll ask me questions if I had a phone or something? What do I exactly do when selling an unlocked iphone that is activated on tmobile? Also what are some starting price suggestions for an iphone 4s 16gb unlocked that's in mint condition, almost no scratches and is working properly with no defects?

Purchase the new phone first. Back up the 4s to your computer or to iCloud. Reset the 4s to its factory settings by doing this through iTunes while the phone is connected to your computer or reset it through the setting menu on the phone. Selling your phone is easy. Take some good pictures of the phone and be honest about the condition when you describe it. Click on sell your item and follow the prompts. Sell the phone as an unlocked T-Mobile phone.
To get an idea of how much to sell the phone, do a search for a "T-Mobile unlocked iPhone 4s ". Look to the left of the screen and click on "completed items". Compare the prices. Sell it by it now or by auction, or a combination of the two. If you choose to auction the phone, start the bidding low so you can get a good group of bidders. If you start it too high, you run the risk of not getting a bidding war started. Start the auction on the weekend completing between the hours of 7pm-9pm