What's everyone else's experience with resale on these? Why are they a hard sell?
There are several factors at play here, I suspect. I'm sorry to say it but I fear the most significant is the fact that you have no feedback as a seller whatsoever. There have been lots of scams online recently, often with people hacking old dormant accounts to post fraudulent FS listings, which is why head-fi introduced the two-factor login recently for selling gear. Do you have seller feedback from eBay or other places? I'd also recommend adding more photos and proofs of ownership. Also, a new seller's first listing being a high-end, pricey item can also raise concerns to potential buyers. Please don't think I'm calling the legitimacy of your listing into question; I realize you've been a head-fier for a long time.
Another factor, sadly, is that the Vérité has lost its shine as an exciting new product, given the subsequent releases of the Atrium and Caldera, not to mention headphones from other brands. I personally think the Vérité remains a lovely and compelling headphone these days, and it does something different from the others in the ZMF lineup, so it still clearly has its place.
The used prices of Vérités have indeed declined in the past year or two, not least because the market has become a lot softer with inflation and perceived potential economic risks in the future. Your £1200 asking price is excellent, especially for a limited version - and a beautiful Blackwood one at that - but that's $1500 USD and other Vérités have gone for not much more than that (or in some cases, for less):
https://www.head-fi.org/classifieds/zmf-verite-open.78861/
https://www.head-fi.org/classifieds/pd-zmf-verite-ambrosia-maple-b-stock.78294/
https://www.head-fi.org/classifieds/zmf-verite-open.77720/
https://www.head-fi.org/classifieds/zmf-verite-open-with-magnesium-chassis.76919/
With that said, I still think your listing is a great value. I sold my pair for $1750, inc. fees and shipping, some 18 months ago; that's roughly $250 more than you're asking, by way of comparison.
Out of curiosity, do yours have a glossy finish? It's hard to tell from the pics. Or do they have more of a satin finish?