Funny that you mention Flintstones, back in the day I was in direct contact with the seller in Germany who had a whole case of them (this is where they all originated from).. long story short I had to sent money in a enveloppe, mail got lost, got found again and got delivered back at my home and in the meanwhile he sold out :'( ... price? €35,-!!!
And yes those box-sets were definitely a way to get out old stock at the end of the Dreamcast lifecycle. I snatched up my Virtua Tennis one from a Portuguese seller back in the day, he had loads of sets for sale, also a combination of PSO + VMU or sth like that if I recall correctly. Too bad I didn't have the funds back in the day to get them all.
Guess the only real hurdles with getting a full white-label collection is getting the F1 red bull demo and the Flintstones one, luckily the majority of high prices/popular one's I already got like Shenmue 1, 2, Skies of Arcadia, Headhunter, Half-Life (video), Sega Extreme Sports Milka, Kao the Kangaroo demo etc