Where to sell old games (collectors' items)

I haven't sold anything in the last decade. Ebay.de seems to be a dead marketplace. Which other sites are there to sell old retail games for more than 5€? We're talking things like old Sierra games, a few Infocom grey-box games and maybe an Ultima 9 Dragon Edition.
Is Ebay.co.uk an option?

It is an option.
My problem with it? It lists also stuff from USA, Hong Kong, etc so I avoid visiting it, I don't want to pay taxes on import - if I wanted to I'd go to ebay.com directly instead of .co.uk.

Ebay is great. I've made plenty of cash selling things on there. Far from a dead marketplace just make sure you finesse your listings properly.

Edit - Oh, Ebay.de is dead, got it. You can likely still sell on Ebay.com - no? There are listings worldwide on the main Ebay site. Try that. I've both ordered stuff and sold stuff to people all over Europe.

Are games still sold on Ebay.com or co.uk as an auction or is it fixed price only now?

You can list things in whatever way you want to. Auction, fixed price, set a reserve price, etc. etc. There are many options.

And go with the better known Ebay.com. You'll get more views that way. Just make it clear that shipping will vary from country to country, etc.

The reason Ebay.de lost a lot of traffic for old games is that it is forbidden in Germany to ship items rated "18" or "not rated" (for example a very old Super Mario Brothers ) to a German address without name and age verification.

You could also check out the Rote Erdbeere (that's "Red Strawberry" for the non-Germans ).

I'm not sure if eBay have fully implemented the ability to sell games without a USK rating yet. That was kind of annoying for people who bought a lot of imported UK retail copies in the early 2000s (like me), because even if the game was Barbie Adventures, the UK version was never tested by the USK, of course, and you could not sell it on eBay because eBay would detect it was a foreign language version sans USK rating and block it.

That's where the Rote Erdbeere was a good alternative because they did not block games based on USK or BPjS shenanigans.
eBay have changed their USK policy a while back thanks to new DHL shipment options so it should now be possible to create listings of retail games that do not have a USK rating.

Personally, I have always gone with eBay in the past. The last games I sold through eBay were some Star Citizen pledge packages and older Collector's Editions (selling non-rare regular editions of old games probably doesn't make much sense since people can get those regular games for pennies on Humble, Amazon marketplace, Steam, wherever…).
I believe you will still reach the broadest possible audience on eBay. The Erdbeere may be (or might have been) nice for indizierte Spiele but if that restriction has been lifted off eBay now, then I'd just go with the market leader, i.e. eBay.

Word. Go with Ebay. If the items have a value, you will get a good price for them by selling with Ebay. In some cases, maybe even more than you expected!

I wonder whether Amazon Marketplace would be a place ? But prices are fixed there ...

Amazon in my experience takes much longer to sell items generally, especially niche or specific items that aren't necessarily trendy or whatever. You have much more of a chance selling obscure stuff on eBay than anywhere else. I've specialized in weird, obscure vintage items there and it has worked great.

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Amazon also has insanely inflated prices for some "collector's items" on the Marketplace.