Is it "safe" to sell a factory reset phone?

[HPstache]

I'm finding conflicting information online.  I bought an Android phone online that did not end up working with my carrier.  I had it all booted up with all my banking apps, google login with save passwords, etc. all on before it was discovered it could not connect for 4G service.  I did a factory reset of the phone and everything seems to be gone, I'd like to return it.  I have read a bit online that the phone is not really wiped clean of pictures, etc because that can be brought back from the memory... but I don't really care about that.  I am just worried about remnants of my passwords and such being brought back from the dead and someone steeling my identity... is that a legitimate concern?  Thanks

[gooki]

Back in the day sure it was a legit concern. Now not so much. Just wipe'n'return.

[reeshau]

Yes, just resetting it leaves it open to someone with a cleanroom.

Just download a memory wiper, and use that before you turn it in.  There are plenty in the app store.  The wiper will fill the memory with garbage, which will overwrite your data.  Make sure to select to fill empty space, too.  It is always important, but particularly since your data is deleted already.  On chip memory randomizes locations in order to extend its lifetime, so even saving the same file can move its location.  Make sure to write 100% of the space.