You should be fine.
When using disk utility, under the erase tab, look at options. You can choose to zero the drive or even do a 7-pass wipe.
Thanks, Im a little scared tho as the wifi on lion is unreliable on all my lionmacs, I hope it doesnt break during installation
Is there any data-eraser that makes my data hard to recover from the buyer? got some personal data
I can't comment on the ability for someone to recover your personal data but the internet recovery downloads the full image first before doing any install. So if your wifi does crap out, you'll be able to try again. It won't go into any actual install until it has a full recovery image.