As of 2026, SketchUp Pro is no longer a one-time purchase. If you encounter third-party sites offering "lifetime license keys" for a few dollars, these are typically unauthorized or fraudulent, as Trimble officially discontinued perpetual "Classic" licenses on November 4, 2020.
The term "license key" can be a bit of a throwback. Since November 2020, SketchUp has largely moved away from serial numbers in favor of Trimble ID subscriptions Modern Subscriptions (Standard for 2026): sketchup pro license key mac