If you are looking to buy a phone on eBay, it would help if you knew what the term "Unlocked" means.
If you are bidding on a phone and the seller says that it is "Unlocked," that means that the phone can be used on any GSM network. All you would need is your SIM Card*.
If you are bidding on a phone that does NOT say "Unlocked," that means that it can only be used with the carrier, or network, that the seller says it works with (i.e.: AT&T, Verizon, T-Mobile, etc.).
*SIM cards are small "chips." They are the key to using one of the modern day cell phones. The SIM Card can be picked up at your carriers' store. Most of the time you can get one programmed for free, but sometimes they charge you from $10.00 to $20.00.