Retired Sets Purchase

I’ve been bit by the Lego bug kinda hard and I feel like the vehicles are more so my thing. Built a few Speed Champions and the Ford GT 500 Technic so far and want to build all the big Technic’s as well for my collection. But after looking in Lego I see that there are a few that arent available for direct purchase anymore. So what’s the best place to buy retired authentic sets(Porsche, Bugatti, Corvette, etc)? I’ve seen some on Amazon and StockX but what’s the best experience you’ve had in buying & receiving some of the more coveted, retired Lego Technic sets? Don’t want fakes either lol. Thanks in advance