I saw someone online comment that it most likely has to do with cloud saves. All the NSO apps have cloud saves by default. Having mainline Pokémon games on the GB/GBA NSO apps would disrupt Pokémon Home, a monetized cloud service that The Pokémon Company runs and profits off of.

It would most likely have to be an entirely separate app that would be connected to Pokémon Home.

I'm not a collector (I open and try to play everything I purchase) so I'm not sure if I have something interesting of value.

After a quick search on eBay and Wallapop, apparently it's a three way tie between my PAL collector's edition of Metroid: Samus Returns, my unsealed copy of Xenoblade Chronicles 2 Collector's Edition and my PAL copy of Pokémon XD: Gale of Darkness without the manual. I've seen prices going from 150 and 220 euros for each one.

The third place goes to the PAL version of Radiant Historia: Perfect Chronology for about 120 euros (not sealed).

And in fourth place we have Metroid Prime Trilogy without the fancy steelbook/cardboard cover for about 80-90 euros.

Besides those, I have plenty of games that go for around 50 to 80 (Pokémon games for example), but I guess those are not that interesting xD

StarCraft 64 PAL. Could easily get over $500 for it.

Super Mario RPG (SNES)

I own Daze Before Christmas on SNES which according to this goes for around $600 these days.

It's a tradition for me and my little brother to play this one every December in the run up to Xmas, so I wouldn't part with it.

Pokemon Soul Silver (Loose)

COD Modern Warfare 3 (2011) Hardened Edition (Factory Sealed)

I don't think I own anything valuable now, but I used to have a sealed bigbox of Carmageddon (PC, 1997) that I sold on ebay for one million euros.

Jesus christ.

Oh. 

Welp, your username makes more sense now 😆

Aside, this thread only makes regret all the more the moment I sold both SoulSilver and Black 2 for dirt prices back in the mid-10's ....

So from only hearsay and a bit of looking through my collection, I'd say my physical edition of Xenoblade Chronicles 2 Torna the Golden Country and also the 3DS remake of Mario & Luigi Bowser's Inside Story are likely my current most prized games. And maybe a few of the 3DS games from Atlus but I'd have to make more research to know.

Edit : 

Forget what I said, my most prized games are all the 3DS Atlus ones lol. Not that surprised though. Torna had a few reprints from VGP and other stores and despite being the lowest selling Mario game ever, 3DS Bowser's Inside Story isn't worth much apparently...