Just reformat the SD. and do a complete factory reset on the Ally from the settings menu.

There is no perfect security, the only way to fully remove data from any electronic storage device is to grind it into powder, and scatter the remains.

 

Do what you said and wipe the Dalvik Cache and do a Factory reset. if you have a computer you can plug the SD card into do a full, not quick format of the card.

 

"Restore to factory default" will get rid of everything on the phone itself. As far as the SD card goes though, if this guy is as good as you say, things can still be read off of a wiped SD card unless you get a program that truly and completely wipes it. Probably best to just keep the SD card and either go buy a new/cheap one and put it in there or just give it to him without.

Read more about it here: (I know this says for HDDs, but SDs work the same way)

http://www.zdnet.com/blog/storage/how-to-really-erase-a-hard-drive/129

 

Microwave! But then i dont think he would but it haha i would do like dautley suggested and do a full format from a pc not a quick one from the phone. And theyre right it wont completely remove everything but i dont think somebody who buys and sells phone on the regular does so to get people personal info. I mean if you have the power to do so id hope you would realize that doing it in that manner would have you bound to be caught.

 

Not to sound like a dick or anything, but if you are concerned about him attempting to gain access to your info, why give him your phone??

 

I like the microwave fix, that sounds interesting King!?!?!