Property purchase & broker behavior: Is this normal?

You can basically try to argue the price down with everything you have. I would do the same.

Of course, you are already pre-triggered by the price since you know it now. Ask around in your environment based on the framework conditions what the property would be worth to them.....
That will give you a feeling for the actual value.
Otherwise, it is quite ambitious to demand such a premium in such a short time....
I myself am currently buying a six-family house at a factor of 14.... and I know, the seller had paid 25% less 4 years ago, he also told me that..... Nevertheless, it is a good investment and the right decision.
Still, once you know the information, you can’t get it out of your head anymore and can no longer judge neutrally.
I would try to determine a neutral market value. Offer this with a slight discount. Everything you have listed here you can use as an argument why this property has this value.... you deal with it.... [bodenrichtwert]. Offer from April, market-standard etc.

Whether you get the bid for it, I don’t know your area at all,... but the difference with you is already quite big.

Thanks Sven!

Value increase is obviously clear and we basically have no problem if someone earns something from it or makes a profit (that's life), but 100,000 within 6 months? That's definitely... special.
I also have another listing from October, it was apparently already tried to be sold for 450,000€.
Apparently, it didn't work out.

So, in the sense of: The tax office does notice that we were already bound from the outset? If yes: How?

From the answers to their questionnaire, which they send to the buyers after the purchase, but it actually seems that it does not affect you.

I made a price proposal today, by email and with an explanation of our reasons. Unfortunately, I made the mistake of thinking that agents can do percentage calculations, so I wrote that we propose a price reduction of/from 15%. In response, I received, among other things, the answer that I had unfortunately not made a price indication....Hm.. Yes, well. That is of course true :D

In addition, we were offered to reserve the property - for €2,600. If we buy, this would be credited to us; if not, the money would be lost. We are now considering whether to agree to such a deal.

It remains exciting! ;)

Business-savvy. That's definitely how you make some money ;)

We also know about reservation fees from some real estate agents. That's a real source of income because most people reserve to secure their financing. If that falls through (and that happens quite often), the agent keeps the money. But reserving without an agreement on the purchase price? I wouldn't do that. That puts you in an extremely bad negotiating position. As a seller, I would hardly lower the price then.