Whereabouts in Europe do you live? In the United Kingdom there's a brilliant second-hand gaming shop called CeX, which is currently selling Diamond and Pearl for £22, and Platinum for £28. And they're even cheaper on eBay - there's some copies available for £10 or less, if you don't mind just getting the game card without the box. I'd be a little bit cautious with eBay though, as some people try to sell fakes on there occasionally; generally speaking I prefer CeX because I know I can trust them.
On the subject of DP remakes, I have a feeling they will happen in Generation 8, on the Nintendo Switch. So far, there have never been multiple remakes released on the same console. Generation 7 seems to be following the same pattern as Generation 5, releasing a pair of "sequel" games instead of a remake.
It's true CEX is my local trade-in store too and they are superb. I know they're scarce in America but in the UK we have it pretty abundant. Pokemon HeartGold and SoulSilver though absolutely trounce Diamond, Pearl and Platinum in value though all of them are relatively easy to find even loose.
EDIT: The thing grinding my gears is people saying the Kanto region doesn't need a revisit and shrugging it off as Gen I wish fulfillment.
Let me tell you I'm playing through LeafGreen right now and let me tell you having played every game since FR/LG are really, really bland remakes. Unlike the others which you know, actually extend upon the lore of their given region and the backstory going on with their legendaries, FR and LG are just polished versions of those exact same games with no extra content. Red, Blue and Yellow got the job done back in the day and I love them for that but Legendaries don't have any significance or presence in this game, their locations are chosen not for any backstory but because they're thematically appropriate.
It's really weird that every single form of Pokemon media including the Origins, XY, HG/SS and the Adventures manga add context to the lore of Kanto but these games barely do at all. The Sevii Islands were a nice little extra but they're barely used at all for anything meaningful. Speaking as someone who loves Gen IV the most Kanto does deserve a revisit, if only to bring the lore aspects and backstory up to the standards of the rest of the series.
I'm beginning to wonder if it was GF's plan all along to leave out a Yellow/Blue/Emerald/Crystal remake so they could re-visit these games at a later date.