Seems to me that you are simply unable to read the help output.
Yes, I tried that already too but same (strange) result, I cant find any obvious error in my command!
root@xp1000:~# nanddump -f dump0 /dev/mtd0 nanddump: invalid option -- 'f' BusyBox v1.21.1 (2014-02-04 22:29:25 CET) multi-call binary. Usage: nanddump [-o] [-b] [-s ADDR] [-f FILE] MTD_DEVICE Dump the specified MTD device -o Dump oob data -b Omit bad block from the dump -s ADDR Start address -l LEN Length -f FILE Dump to file ('-' for stdout)