Heckin Steve described it well. They likely charge double market value because of the warranty - if they don't have it in stock and need to replace it, they'll likely buy another one at the ebay/pricecharting price.
I own 56 PS2 games, maybe 10 of them I bought in person , 2 don't have original case or any artwork. I remember 2 years ago I was going to order from Dk Oldies, but then I matched up the same games and ended up getting like 5 or 6 games more for the same price. I have spent 20-50$ on a few games that I had to have. Other than that every disc, case, artwork, overall quality has been an 8.5/10. Every disc i got from them was clean, not scratched; even if it was put on a resurfacing machine... there was no residue. Every game has worked except my old NFS Underground 2 that was from a shop called V Stock in the mall. Dk Oldies started business in 2004. Lukie Games in 2010. Oddly the websites are similar too. I also bought a Ps2 Fat from Lukie Games in August 2024. Console was super clean, few minor scratches, but the inside was a clean as a whistle. It was 200$, but wayy worth it. I broke my fan connecter on my motherboard on my other Fat Ps2, thats sitting in a box, labeled, and ready to put back together. Havnt been ready to tackle it just yet. Fat ps2, Defender Bluetooth Controller, Levelhikes hdmi converter, and a 128mb memory card is all I need. No internet, no downloading, no updates, no subscriptions, and no having to uninstall games for new games. Yes, ps2 emulation is getting better and better, but just having a physical copy of games from 20+ years ago is just satisfying to own.
I have bought from both of these sites and sometimes I just don't want to hunt and don't mind paying a premium. I overpay for games at my local games stores too if it is a game I want and they have it in stock. I like supporting stores and the people who work in them.
I want them to stay in business. I have plenty of disposable income and enjoy their services. I like when people have jobs. It would be a shame if they went out of business.