Any place offering to sell you a "gently used" iPhone for $100 is probably scamming you. If it even is a real iPhone, it will like be gently beaten to a pulp, with scratches on the screen and lots of wear on the case. The battery will be half way through it's life, which is another huge expense (it will cost you more to replace an iPhone battery than the $100 bucks you pay for the phone, and you WILL have to have the battery replaced). In all likelihood, this is a scam of some type.
Even if it isn't, you can't use an iPhone on your service anyway. Alltel and Verizon use CDMA networks and phones. The iPhone is a GSM device which uses a SIM card. You can't use that on your network. This is why everyone with an iPhone is using AT&T, since they're the only network with full 3G support for the phone.