Revert Firmware on ACM3200, remove all personal data

Apparently the ACM3200 has two firmware flash partitions

Dual Firmware Flashing

The WRT AC series of routers uses a dual firmware flash layout. This means that two separate firmware partitions are included on the device and are flashed in an alternating fashion.

If booting from the primary partition, the secondary (or alternate ) partition will be flashed on next sysupgrade. The reverse logic is also true. See the Flash Layout section for more details.

I've just flashed the OEM back on to it using sysupgrade -F- n , but do I have to do anything else to wipe all my data off other partitions before I sell it? My feeling is no as it's only the flash partitions, but just wanted to confirm. Thanks

If you have only flashed it once, the other partition contains your OpenWrt data. So, you should flash it once more so that both partition have been overwritten.