Being 2017, the iPhone 6S is still a viable option for purchasing and easily recommended. Do note that if you buy a used iPhone 6S, it will likely be unlocked at the time of purchase.
Compare: Used iPhone 6S (Unlocked) prices
With your phone unlocked, you'll be able to activate your iPhone 6S - as long as you have the necessary SIM card, anyway - with a carrier that will accept the device. If you're unsure of which cell phone plan you should opt into, you can always refer to our cell phone plan comparison tool.
Additionally, if your refurbished smartphone is not unlocked, then you should refer to our guide, "How to Unlock Your Phone & Use it With a New Carrier."
Nevertheless, in regards to the iPhone 6S, the phone is not too large or small - in fact, one could even say that it is just right - and is considerably comfortable to hold. The phone is equipped with a 720p resolution and 4.7-inch display, which isn't exactly fantastic when compared to the Samsung flagships, but it shouldn't harm your viewing experience.
Furthermore, the A9 CPU and 2GB RAM keeps the iPhone 6S running quite well, even when compared to the iPhone 8. You will be able to run apps with ease, and you won't suffer slow down when upgrading to iOS 11.
Pertaining to the battery and with a typical workload, the 6S is able to run - without the Low Power mode activated, anyway - on a single charge for about 13 hours on average, though it can range between 12 and 14.
However, what really makes the iPhone 6S shine is the 3D Touch, which detects how hard your press is on the iPhone screen.
As far as the camera goes, the iPhone 6S does not have optical image stabilization, which prevents shaky hand motions from distorting any photos. However, you'll still be able to shoot detailed photos, but when it comes to low-lighting, the iPhone 6S suffers. However, you will be to shoot 4K video at 30fps with this handset.
Overall, the iPhone 6S deserves praise for its ability to remain strong despite the current smartphone market. Not only does the phone remain efficient and fast, but it also is able to provide you with features that are becoming increasingly demanding, such as 4K video recording. If you're looking for a used iPhone 6S at the best price, you should certainly search for prices online.
You'll find iPhone 6S models available for sale at a cheaper price online compared to what you would find at a physical retailer. However, it's also important, before you buy, to compare prices of refurbished and unlocked iPhone 6S models from the top resellers to ensure you'll receive the best price.
However, if you're not interested in buying a used iPhone 6S and instead are looking to sell your iPhone, then you may want to read our article, "Where Can You Sell Your Old Phone?" Additionally, to earn the maximum amount possible, you should read our guide on selling your iPhone 6S.