I purchased a Sinn 156 Military from a private seller in Germany a year or so ago and it went flawlessly.
My (limited) experience with chrono24 thus far has been that private sellers are much better to deal with than the so-called 'trusted sellers' who advertise on the site. Most of the latter either a) don't bother to reply to inquiries b) respond with 'the watch has been sold' or c) are grossly overpriced.
On chrono24, as everywhere, buy the seller. Get good photos (with the watch set to a time you request, although chrono24 already does something like this for private sellers), speak to the seller if possible, and go with your gut feeling. If anything seems 'off' then move on. Finally, pay with PayPal or some other payment service that provides buyer protection.
Good luck.