Normally I'd say you were wrong...if a store clerk gives you the wrong change you should bring it up.
But in this case you offered a price and he accepted. It's up to him to look up the retail value and decide if it's worth it. Either he did and was ok with that price or he didn't in which case that's his fault.
You're fine UNLESS you're leaving out a part of the story where you told him it was an iPad 1 and only worth that much and he should sell it quickly 'cause the second version is out already. In that case you'd be wrong. But if you only thought that and didn't say it out loud then it's not your problem.
I agree with Small White Car, as long as "you gave all the details". You also did not know for sure if it worked!
You made an offer and he accepted...