From a legal standpoint, he is technically holding your stolen property and you could demand him to return it to you, but from his point of view, he doesn't want to go that way as he would want to return it to the seller he bought it from, so that he can get a refund.
Theoretically if you wanted the goods from the german seller, you would have to provide evidence to the German Authorities that these stolen goods are in fact yours. There is no locking the stuff up in evidence or anything, they will be directly return to you. That would however be a very lengthy and complicated process and before that will has happened, the German seller will have already returned the goods to the spanish seller.
What you now must do is contact the police in Spain and tell them the dealer in Spain is now holding your stolen property and you want to have that returned to you. You provide police with evidence that the items belong to you and they will inform the seller to return them to you.
Nothing more will come from the case. Police is useless and their time is better spent writing parking tickets and checking if someone has all their reflectors on their bicycles than solving real crimes.