mkdir -p /srv/pacman2pacman/torrents
chown -R transmission /srv/pacman2pacman/
pacman -S transmission-cli
/etc/pacman.conf:
  XferCommand = /usr/bin/pacman2pacman-get %u %o
cp pacman2pacman/pacman2pacman-get /usr/bin/
chmod +x /usr/bin/pacman2pacman-get
chmod 777 /var/cache/pacman/pkg
systemctl start transmission

If you want to see the packages being seeded, run: transmission-remote -l

BUG: if you changed your package cache location (it's customizable in /etc/pacman.conf) you might be doomed. pacman2pacman assumes it is /var/cache/pacman/pkg