So basically...?
1) The Nexus 7 doesn't use the FAT file system so stuff is essentially erased
2) If the government or something with a lot of resources wanted to get the erased data it will be possible
3) That person is not going to get the Nexus 7 I send back
4) See 1, 2, 3 and do a factory reset and don't worry
I looked at the erase programs like "forever gone" etc and they didn't have very good reviews or just erased SD drives.