Is a broker commission payable if the intention to purchase was known beforehand?

That doesn't really help if you have no idea about buying a house in that area. Anyone can call themselves an "expert" or which certificates should be looked for, etc. The good old yellow pages don't exist anymore, and digital searches are very complicated since there is no single point of contact for such searches.

Where can you find neutral appraisers for property valuation? Are there lists sorted by postal code? I would recommend that to the seller.

For example, Sprengnetter will surely be able to provide you with such information.

That doesn't really help if you have no idea about buying a house in that area. Anyone can call themselves an "appraiser" or which certificates should be looked for, etc. The good old Yellow Pages no longer exist as such, and digital searches are very complicated because there is no single point of contact for such searches.

And your bank also knows about buying a house: they not only tell you how much money you are generally good to borrow from them, but also whether they would specifically lend you that much secured by this particular property. By the way, the "Yellow Pages" still exist. They are just white now and usually appear at the front or middle of the local / "official" phone book.

Anyone can call themselves an "expert" or which certificates should be paid attention to, etc.

Anyone can call themselves "expert" and "appraiser" according to their own liking, but "publicly sworn" rather not. The chambers of industry and commerce (IHKs) examine and name such persons. And maybe you can also find out from the auction notices in the district court hallway who appraised the objects there. Or ask the court clerk whom they use (maybe not official, but they also go to the canteen...).

Can anyone call themselves an "expert" or which certificates should be paid attention to, etc.

Well, on MyHammer you shouldn't look for him. I would have also thought of the IHK. I wouldn't go it alone here without the homeowner – ultimately, you have no rights at all. However, you are also in a bit of a dilemma: Normally, as a buyer, you get an expert so you don't buy a pig in a poke. You both have different interests – according to the report, he wants to get as much as possible, you want to pay for a good price-performance ratio if possible. He takes his broker for the value analysis because it's free (in your hope, without ordering a commission), you take one from the IHK, who determines the defects and renovation costs. Do you actually talk to each other? I mean, are you in good contact? Then I would also communicate openly that he should wait with a broker assignment.