Dear LR,

I would say this is not so much about trust in the seller, this is rather about trust in the item. As you didn’t share the link, it is impossible to evaluate whether the seller is reliable. Thus, it might be a good idea to edit your question and share the link to this auction. In any case, the mere fact that the seller has been selling alike necklaces in the past does not mean anything. A seller can have several similar items available. That’s perfectly ok.
I believe, the main question here is whether these pearls are real. Unfortunately, it is quite difficult to tell for sure on the basis of this single image. You have to look for the growth marks. Natural pearls always have growth marks. Very high-quality pearls have very few growth marks but still, they have them, and a professional seller will take images showing them. Faux pearls, on the contrary, have no growth marks. So again, it might be a good idea to share the link to see more images.
The clasp in itself does not prove anything. I have seen both Akoya pearl necklaces and faux pearl necklaces coming with clasps like this. JKA stands for German company J. Köhle GmbH. To the best of my knowledge, they have been manufacturing both real pearl and faux pearl necklaces in the past. More importantly, it is not uncommon that pearl necklaces get re-strung. Thus, this is not necessarily its original clasp.
To sum it up, the first impression is that this is a legit Art Deco Akoya necklace. However, more close-up images are needed to tell for sure.
Last but not least, do not rely on the price estimates given by Catawiki’s experts. These estimates are highly inflated (especially for items with reserve prices). I have been following thousands of Catawiki auctions, and there is always the same pattern – the true market value is 20 – 30% of the estimate. This case is not an exception.

Best regards, Editor