Well I think 0.03 BTC is fair enough with that accounts activity. Signed message proves ownership od the account. Here is the guide if you don't know how to sign one.https://bitcointalksearch.org/topic/how-to-sign-a-message-990345
It's too late now. Assuming that he doesn't know about signing messages (from the fact that he didn't know what the term was), he hasn't staked an address yet. A staked address that is posted a couple hours/days before an account sale is worthless.
he probably doesnt know the "signed message" term but he can possibly posted an address when joining signature campaign or maybe on claiming giveaways, he should know how to sign a message first before we assume that he really dont have any address posted from the account he is selling