Waste deposits on property

That is then your personal bad luck! But then you should never start building a house;)

That's why I said earlier, you can never be 100% sure. :P

If an expert says: clean soil, then the soil is clean! You have to have some faith in the abilities of the experts / professionals, otherwise the house construction will not work. I wouldn’t worry anymore if you buy a demonstrably clean property. You even have the advantage that the investment is borne by the sellers - something like that is rare nowadays!

You could choose an expert yourself and commission them yourself, questioning according to which pattern they take which samples and which analyses are carried out.

In addition to the soil report, you could commission an analysis of the "white powder."

IMHO -- without knowing the specific circumstances -- you are being overly cautious (me saying that in this context ...). You can trust an independent, solid expert! If they find nothing, there is nothing.

If properties in the neighborhood are contaminated, contamination can of course migrate or spread over the years (while being diluted). In this case, a report on your property today will not help. So next to a gas station ...

Another question:
Do you want to buy a property in an area where people dump their trash? What kind of people live there? Aren't there better areas?

The expert is now making an expert report. In the report, he confirms, for example, the absence of contaminated sites and clean soil. Since I don't know how to verify such things, my question is whether, after the report, one can really assume that the soil is clean, or if there always remains a residual risk because such things cannot be checked 100%?

He will not sift through your entire property and analyze every single grain! He will take soil samples – how many, I don’t know. Certainly in a grid pattern from which one can then infer about the entire property. At least enough so that an educated person with expertise can create a report that will be indisputable in court.

So let’s assume – free of contaminated sites and clean soil. But if after a few years traces of, for example, gasoline or oil are found, can I then hold the expert accountable because he had confirmed and assessed the absence of contaminated sites and clean soil back then?

No, of course not. Your mother-in-law can, after the basic soil survey, contaminate a few spots with her rusty shed ;)

Of course, traces of something (gasoline or oil) can be found, but "traces of..." do not yet constitute contamination damage. Therefore, a small contamination from the residue of a 5-liter gasoline can might not necessarily be detected by a sample because that soil area simply falls outside the grid. If the toxins from these residues have not widely and concentratedly contaminated the soil so that contamination can be detected, then there is no contamination in the sense of a "contaminated site." After all, there are so-called threshold values that evaluate "traces of..."

I do not know any contaminated site reports myself. Nevertheless, there will probably not be a two-liner saying "there is nothing here," but rather a multi-page report, in which samples are compared with threshold values, and stepwise contamination is shown (if present) – with a conclusion on whether the property is considered "polluted" or not.
I would be very surprised if a leftover of roofing felt and an empty canister turn the property into a contaminated site dump.
Be glad that the current owners are paying for this report :)

I have negotiated a 2-year right of withdrawal in the purchase contract of my property (which was previously partially used commercially, carpentry) in case of contaminated sites.
In other words: If we find plutonium when excavating the foundation pit, he gets it back.
But it is only a safeguard for really severe contamination.