Hey, man... sorry to say this, but if you had all that, you should definitely had claimed the land by now. You probably focused too much on other things.
As go negotiating, first you would have to find the person online, then you would have to see how reasonable the person is. I hardly doubt he would just let you have the stuff back, since he stole it in the first place. But I suppose it's always worth trying.
Good luck. If you can't get that back, just move elsewhere and rebuild. Building stuff isn't that hard, the really hard part is finding a nice location.
EDIT: You could also leave a runestone telling the guy it was your stuff and all... or giving a HS for him to add and talk to you, but I wouldn't keep my hopes up.