ESN = Electronic Serial Number
As suggested, when buying from Ebay, check out the seller's feedback and make verify that the ESN is clean and not stolen. If you ask the seller, they may let you know the ESN so you can call Sprint to verify that it's clean.
Do verify the description. Make sure there is no place on the add stating "Photo only", so they don't scam you by you thinking you're buying a phone, but actually you're buying a photo. If you think that's dumb, check this video out of a
scammer trying to sell 2 sell phones, which are actually just photos.
Sprint or any provider will let you activate any phone, just as long as it belongs on their network. You just have to call them and let them know the ESN.
I love my 700wx. The keyboard on front makes it easy for 1 handed operation.