Until there is a better solution…
When I deleted the /var/tmp/pamac, pamac recreated the directory, as:
pamac drwxrwxrwx root root
pamac/dbs drwxrwxrwx root root
pamac/dbs/local lrwxrwxrwx $USER $USER -> '/var/lib/pacman/local'
pamac/dbs/sync drwxrwxrwx root root
pamac/dbs/sync/core.files -rw-rw-rw- root root
pamac/dbs/sync/mhwd.db -rw-rw-rw- root root
pamac/dbs/sync/core.db -rw-r--r-- root root
pamac/dbs/sync/multilib.files -rw-rw-rw- root root
pamac/dbs/sync/community.files -rw-rw-rw- root root
pamac/dbs/sync/multilib.db -rw-rw-rw- root root
pamac/dbs/sync/community.db -rw-r--r-- root root
pamac/dbs/sync/pamac_aur.db -rw-rw-rw- root root
pamac/dbs/sync/extra.db -rw-r--r-- root root
pamac/dbs/sync/refresh_timestamp -rw-rw-rw- root root
pamac/dbs/sync/extra.files -rw-rw-rw- root root There is a caveat though. pamac gave me the error below, but still copied community.db, core.db, extra.db, multilib.db and created refresh_timestamp. I responded “N” to install and ran the command again. This time no error and these files were created: community.files, core.files, extra.files, mhwd.db, multilib.files, and pamac_aur.db.
Note: Prior to the removal, everything was owed by $USER.
cp: cannot create regular file '/var/tmp/pamac/dbs/sync/community.files': Permission denied
cp: cannot create regular file '/var/tmp/pamac/dbs/sync/core.files': Permission denied
cp: cannot create regular file '/var/tmp/pamac/dbs/sync/extra.files': Permission denied
cp: cannot create regular file '/var/tmp/pamac/dbs/sync/mhwd.db': Permission denied
cp: cannot create regular file '/var/tmp/pamac/dbs/sync/multilib.files': Permission denied
cp: cannot create regular file '/var/tmp/pamac/dbs/sync/pamac_aur.db': Permission denieI normally use pacman and pamac for AUR.
This probably should be reported as a bug ? 
Same issue: