They seem to have a decent selection and are suppose to guarantee the phone has a clear ESN# so it can be activated. Also, they require pics of the actual device. There is always Ebay but you don't always get the truth about ESN#'s from those sellers.
You could also look in the iMore Marketplace. It's part of Mobile Nations (CrackBerry, Android Central, etc.). I didn't see a feedback forum at first glance, but you can see how long the seller has been a member.
I’m glad to see that you are interested in getting an Apple iPhone 4S! It’s one of my favorite devices. It's SUPER easy to use and has a great build quality! I have great news for you! Since the launch of the iPhone 5, the previous iPhone models have had a price reduction. The iPhone 4S 16GB is now on sale for $99.99 with a new 2-year agreement. That’s a HUGE savings from the former retail price of $649.99! To check your upgrade status, simply call #UPG from your current Verizon wireless cell phone. If you are interested in shopping for various memory size and colors then here’s a link for you. www.verizonwireless.com/iphone. Happy shopping!
Follow us on Twitter @VZWSupport
Ebay is a good place to look and you have buyer protection incase they don't deliver item as described. Ebay will get you your money back and most sellers will work with you. I usually go with a seller that has a lot of positive feedback, because they don't want you to post something negative. Ebay/Paypal also holds your payment until you receive the item and will release after a period of time. Quicker release if you give positive feedback. The only thing is that, if you do have to return an item, buyer usually pays the return fee. You do get your original price plus original shipping fees refunded though.