Have you tried rebooting the laptop in case the second time is the charm? Once in a while I encounter something like this and a reboot fixes the problem.
Did you perform the clean install, or did the previous owner? You may want to perform your own clean install of macOS by first erasing the whole physical SSD before reinstalling macOS just in case the previous owner didn't prepare the Mac properly. Within Disk Utility you may need to click "View" and select "Show All Devices" so that the physical SSD appears on the left pane of Disk Utility. The physical SSD should be identified by the make & model such as "Apple SSD....". Select the physical SSD and erase it as GUID partition and APFS (top option)...assuming you are reinstalling macOS 10.13/10.14+. Quit Disk Utility and select the "Install macOS" option.
To reinstall macOS, boot into Internet Recovery Mode using Command + Option + R to access the online macOS 12.x Monterey installer (hopefully). Unfortunately sometimes the Mac may only boot into the macOS online installer for the OS which originally shipped with the laptop from the factory. For a 2015 laptop this will be macOS 10.11. If this happens you will instead need to erase the physical SSD as GUID partition and MacOS Extended (journaled).
Just to confirm that the builtin trackpad does allow you to scroll the EULA? Can you also click on other items to interact such as the "back" button or "decline", or a menu? Maybe try using an external keyboard and mouse just in case there is an issue with the laptop's builtin keyboard & trackpad (a common problem with the 2015 MBPro, but usually both the built-in keyboard & trackpad will stop working at the same time).