Question regarding an Iphone 6S Plus

Stay away from it "as if it were a swarm of bees." Read: The All Too Common SAD Reality of Buying a Used iPad/iPhone - Apple Community for further details.

Ask Ars: What are those symbols on the back of the iPhone? provides a summary of what the symbols mean. As stated in the article:

"This only covers the iPhone that is authorized to operate in the US, Canada, and Europe. iPhones manufactured for distribution in other countries may have a completely different set of certifications printed on them (some devices, like the Kindle, slap every regulatory stamp they get on the back so they have one version that can ship anywhere). All the rest of the iPhone's compliance medals that we didn't discuss here can be viewed in the device's Settings menu (General > About > Regulatory), along with the names of the countries that use them."

I would also emphasize this paragraph from the first article I referenced:

"The Bottom Line:

1. ONLY Buy Apple devices from trusted sources - An Apple Store or an Apple Authorized Reseller
2. If you can't afford a brand new iPad/iPhone, Apple and many Authorized Reseller's sell refurbished devices at reduced cost. These devices are often indistinguishable from a brand new device and they come with warranties.
3. Save your money and wait for a major holiday sale, where new devices are often discounted.
4. Don't become a victim. You worked hard for your money. Don't take the chance of being ripped off as so many are every single day. "