House demolition - What to keep/sell, what to dispose of?

I would spontaneously think of the double-glazed aluminum frame windows, or the brick of the masonry. Also conceivable would be doors and frames that also date from the period of construction.

Aluminum windows are worth removing for reuse if they already have separate inner and outer profile halves – those with one-piece profiles (mostly brushed) can practically only be melted down. But one way or another, you can learn a lot from dismantling them that will be useful when installing them. Brick is very popular; it will still be the old Reich format. It is often used for cladding and for kitchen counters, wine cellars, raised beds, and the like (or for repairs during renovations). But hand it over as a lot to a specialized dealer – otherwise, you will need a barn for temporary storage and constantly have people calling who only want small quantities. Profiled frames, doors with panels (especially glazed at the top and wood in the middle and bottom) are also well regarded, but: see bricks.

I would only want to take something down and keep it for myself. For the rest, hire a company. That is more effective and safer.

and what do I take with me?
There are the beautiful doors, which could certainly be repurposed or used as sliding doors.
Bricks and paving stones can be used as striking eyecatchers or simply outside as flooring or garden bed borders.
Maybe also a piece of terrazzo flooring, which could later serve as a side table surface.
I also think light switches are brilliant.
What about laundry sinks?
It always depends on the taste one follows. Old combined with new is always a nice decorative thing.

There were the beautiful doors,

Nice, yes - but from the description I read that the OP was a tall lanky guy, and in 1928 they probably hadn’t installed 213 doors yet.

...a friend would be really crazy about such things. He even had 175 high doors installed again and Bakelite rotary switches... But he is crazy ^^

I think I recently read something about companies that specialize in old building materials. I'll reflect on it when I get the chance

There is a recycling center in our region that purchases or accepts on consignment such components, etc. The demand for such parts is huge. There are plenty of construction projects involving monument-appropriate renovations or builders looking for beloved details. They are not recyclers! They are dealers who are aware of the significance of these treasures. To what extent it is financially worthwhile now is certainly a matter for negotiation.