IMEI unlocking a world phone, particularly on CDMA networks like Verizon and Sprint, are really not worth it to be extremely frank. You're better off investing $20 in an interposer, and slipping it between your SIM card and your SIM tray and just roll with it. It supports 99% of carriers worldwide and there's no risk for getting scammed with an interposer as the profit margins on them are extremely high for the seller/reseller, so there's no reason to not ship the product as advertised to you.
Always check the reputation of an unlock seller prior to purchasing. With smartphones being in almost every person's hands or pocket, it's an easy money maker and easy scam, particularly CDMA carriers like Verizon and Sprint on the 4S and newer devices. The best method of payment when purchasing online services is to do so through Amazon Payment, Google Wallet, or through Paypal, that way, you can dispute the charges when it goes sideways.