For those unfamiliar with Catawiki: it is a platform. It does not assume any responsibility for misrepresented items. Although it does withold payment from the seller and refund it if the buyer can prove the items are not as described.
Catawiki does not hold the items, nor does it examine them. It is not much different from Ebay, The auctioneers, however, are experienced. In the past, I noticed the auctioneers for one geography suddenly compiling the auction for another geography.
I, regularly, look at the GB, Spanish, and Dutch stamp auctions. I observed the British auctions tend to offer very bad items that command too high prices. Dutch and Spanish auctions appear to offer better quality.
Catawiki, as the name suggests, started as a catalogue wiki for collectables. It incorporated an auction. These split and the catalogue section, now, is called LastDodo. This is an apt name as the people administering the stamp catalogue and sales part of LastDodo have little to no philatelic knowledge. Some consider themselves expert when they own a Michel catalogue or have half a DAVO album filled. Others consider themselves experts on philately because they collect comics. They re-invent philately and the discussions on the stamp forum are absurd.
I am not sure whether people behind LastDodo remain those behind Catawiki, but I notice there is some relation. One example is that if you offer an item above a certain price, they will redirect you to Catawiki.
There are good deals to be had but you might want to check whether any of their auctioneers has experience with US stamps.
Online references to 'Mark Fodden' lead to the owner of Northern Stamps that offers GB and Commonwealth stamps:
https://www.northernstamps.com/. And if you have the audacity to offer this heavily mutilated stamp that has the inverted watermark as the normal stamp at £ 150 ....
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