In the early 00's a lot of Dreamcast stock got liquidated. You could buy stacks of sealed Dreamcast games for €1,- pp as a consumer, so just imagine the prices being paid at wholesale. From what I gathered JoshProd/Pixelheart got their stock from that era and has kept it under wraps until it was financially lucrative to start selling them.
There seemed to be a lot of NOS European games as recently as 2015 or thereabouts. There were sellers who were scrapping the manuals and discs for common games like Metropolis Street Racer and Chu Chu Rocket and selling 10 packs of replacement PAL disc cases from them!