First Gen iPhone data wipe

[DuckSoupe]

I recently purchased a first generation iPhone to use as a data device (basically iPod touch) in conjunction with my iPhone 4. I wanted the first generation iPhone instead of an iPod touch simply because it is my favorite of all the iPhone designs. However, I digress, the iPhone was not working properly or in the condition that the seller described. I have since been issued a refund for the purchase and am in the process of returning the device to the seller. I attempted to wipe all personal data from the phone (contacts, apps, music, and itunes account) by going to settings -> general -> reset -> and erase all data and settings. After performing this operation the iPhone will no longer boot. It will turn on, get to the Apple logo screen, and then freeze there until the battery drains and dies. My question is, am I safe in assuming all the data and information is off the phone? Would there be anything left on the device, or anything that I can do to be sure that all the data is gone? I apologize for the length of the post but I wanted to be sure I was clear in what I was asking. Thanks to all in advance for any replies.