Of course the standard internet tough-guy response would be: Drive over and beat him up. But in the real world that would probably not lead to the desired results.
Unfortunately I've had to deal with a few similar issues in the past. How did you pay? If Ebay, Paypal or a CC were involved, there's a good chance you can get your money back from them.
I'm not familiar with Finish laws, but over here it's crucial to set the seller a time limit by which the goods have to be delivered. If it's the same over there, do that (by registered mail) and make it look somewhat official. I usually use the last names and address of my girlfriend and her room-mate and make the head of letter look lawyer-ish. Can't hurt to enlighten him about the consequences and quote the respective laws, too.
In my experience it's also very effective to make a big stink about this in his direct environment. Call his neighbours, employer, bank, parents, football-teammates and anyone else you can find via Google or phone books and ask them for other contact information. Also mention that the subject is owing you money, that you're gonna get the police involved and that they should relay this to him since you're unable to get a hold of him. Tell the other victim to do the same. If he's leading a halfway normal life otherwise, he won't want to lose his face in front of his friends and family, and they'll start to inquire, especially when hearing from more than one person.
If you do get a hold of him, give him an "easy way out". He's got to have an opportunity to pay you back without admitting to be a thief (unless that's painfully obvious already), because that won't happen.