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
