I did some extensive searches... most, if not all of the listings I saved are still up... but not shown in search results. They are purposely suppressing the searchability of these items...
But they do exist.
Why did eBay suddenly start giving a damn all of a sudden about Mercury lamps?

I wonder if they're trying to get the low hanging fruit but restricting visibility of listings containing the word "mercury". It seems like other lamps with mercury such as HPS, metal halide, and fluorescent aren't subject to this restriction for now. Even listing a lamp as H39 bypasses this.

Around a couple of years ago, eBay started cracking down on shipping items that they deem (think) contain mercury internationally. The reason they were able to do it is that through the past decade, eBay has been convincing US based sellers to use their Global Shipping Program instead of shipping directly to the customer. Instead US based sellers ship to a eBay facility in Kentucky which sorts and then forwards it to the final destination. In theory this was supposed to allow US based sellers to sell internationally and provide international buyers with easier access to US eBay listings. Instead I found shipping rates never really decreased and it was cheaper almost always to ship to a 3rd party that brings the item over to Canada.

By doing so, eBay also has the abilty to filter out any items they deem unsalable. There are reports from 2 years ago that any HID lamp, as well as anything similar such as vacuum tubes being confiscated at the Kentucky facility and both the seller and buyer refunded. The item of course stays with eBay, it was later revealed that a lot of confiscated items would get resold on eBay under a 3rd party, often copy and pasting the listing and pictures from the original seller. Now I'm not sure if eBay still does this practice but there's a good chance they still do.   

 I'm seeing a lot of MV lamps still available on eBay . They're just alot harder to find . But they are still there !

Most of what I see on eBay I wouldn't spend money for considering the build quality...

It is illegal in some states to sell mercury vapor bulbs, as I understand it. I have noticed on Facebook Marketplace that a search for "mercury vapor" yields 'no results', however if I search for "street lamps" I can find listings which are titled "mercury vapor streetlights". So it seems there is some filtration going on.

I assume that Facebook and eBay filter out these items as it is illegal in some states to buy and sell - even old stock. From the platform's point of view, it is easier to suppress all posts to appease the localities that do not allow sale, than risk allowing a sale which could cause the company consequences.

As an aside, I see NOS mercury vapor fixtures pop up from time to time on Marketplace. What is a fair price for these? One posting I have been watching wants $95 for a "Charleston"-style outdoor MV fixture. It is very interesting, but I bought another NOS MV fixture (NEMA style - bulb was not new, and reflector was cracked) for $40. Perhaps that has skewed my frame of reference.

Most manufacturers have since discontinued mercury vapor bulbs. GE and Feit seem to be exceptions. You won’t even find them under an alphabet soup brand on Amazon. I believe Sylvania and Philips discontinued the MV lamps in like 2018 or something.

I believe GE still has mercury vapor bulbs (obviously they don’t manufacture them anymore).

I was under the impression that MV bulbs and ballasts are still used in China and are still under manufacture, but not sold in the USA due to bans.

The china-made stuff is not great quality but something is better than nothing... I see that some MV bulbs seem available on alibaba. I wonder if that would be a way of getting some. Maybe the technology is old enough that customs would not know it is banned 

In future, it will make more sense for members to swap or sell more MV lamps amongst themselves.

Depending on personal interest, this can be done at a national or international level.

You’ll just have to accept the postage costs. When swapping or selling in this way, it makes sense to post several lamps at once.

There’s a section for this on LG …

There's no ban or anything affecting mercury containing lamps and shipments. I recently bought a few hundred T12 fluorescent tubes on ebay without an issue just a couple weeks ago infact. I was able to find a seller who sells 1200mm length 40W tubes for very cheap so I bought a few boxes

I think listings just come and go.

It depends on where. The legislation about old items not meeting new standards varies a lot across the world.
In Europe the general rule is once it exist inside of the European union, you may sell and resale it freely as you wish, the laws just ban the manufacture and import. There are really extremely few exceptions of this rule, mostly related to things like weapons or drugs, something what is deemed really extremely dangerous. But not something that just stresses the environment. Moreover with things like the products containing mercury or other toxic materials that complicate disposal, it does not make much sense to ban the resales if the thing is already here, it would need to be disposed of, so cause the environmental stress/damage either way.

But in the US it is quite common when some laws want to regulate some product, they just plain ban any further sales, even by individuals, not just manufacture/import.

And I would not be surprised the same Ebay will have no problem allowing these sales within EU, but not in the US.

If you find one at a local store, most likely it will be modern junk and not be worth buying.

depends on the price. A bulb that only lasts 3 months to 2 years but is on clearance for 3 dollars is definitely worth it. I wouldn't buy the cheap crap for 20+ tho.