Ensure the listing explicitly says "Unlocked." A "locked" phone is tied to a specific carrier (like AT&T or Verizon) and won't work with other SIM cards unless the previous owner paid it off and unlocked it.
Before you buy, keep in mind that the iPhone 6 is very limited today: can you buy an iphone 6 without a plan
Specialized sites like The Big Phone Store often sell older models that have been professionally tested. Ensure the listing explicitly says "Unlocked
If you need a cheap iPhone that still supports modern apps and current security updates, consider a certified used iPhone SE (2nd or 3rd Gen) or an iPhone 11 , which are much more capable in 2026. can you buy an iphone 6 without a plan