Free version limitations
The second-hand version of TrapTune loses several knobs that would give you access (for example) to detailed settings of the particular effects sections while you can still turn reverb, distortion and delay on and off as you wish. Furthermore, the free version can load the full version's presets and settings (which is helpful if you, for example, load a session made with the trial version or when you share projects with people owning the full version).
How to get it?
There is no special installer for TrapTune's free edition. Just download the full version's installer from the product page and explore the 15-day trial version. If you decide not to activate, TrapTune will switch to a limited version when the trial period expires. Get the trial here.