You don't think, that's the problem. Stick around and maybe read a little more. You might be surprised. I thought I did but I didn't? Damn.
The issue I see here is that you grabbed the domain with the full knowledge that you intended to seel it back to them (or this is how it willbe seen as you contacted them not just used it), therefore the lawyer will be able to win the argument. Sell them the domain at cost, admit no wrong. Have you already solicited a fee? if yes appologise as a miunderstanding if not just state you saw some advertising and realised they may have expired it by accident and reiterate no wrong doing and a release at cost. Otherwise it could cost you an arm and a leg in legal costs which you wont win.