Nope, best advice we can give you as a
DJ site, is to NOT spend your money on CDJs and analogue mixers 😀
Seriously though, it’s an investment in obsolete technology. Especially the higher end CDJs (and you don’t want a set of used 400s) were hardly ever in the private domain, meaning anything second hand out there has been used in venues, usually long and VERY hard. Always by people who’s gear it wasn’t, so the amount of careby users would have been lower than they would have given if it WERE their own.
I’ve seen DJs smashing buttons on CDJs that you wouldn’t believe. Couldn’t have done it harder if they’d used a hammer. I’ve seen drinks spilled over gear, smoke (both the DJ kind and the tobacco kind) and oil crackers (hazers) running all night every night (and that stuff DOES get into your knobs and faders). All stuff that eventually leads to often invisible damage.
So if you are still set on going (way) back to analogue, I’d say getting it from a reputable company who traded them in, had them refurbished and who are willing to give you some warranty on them would be your best bet. I know Pioneer themselves have a refurbished offering (albeit limited) on their site somewhere. Buying from other (online) sources would scare me more. I’d have to be able to see it play, play with it myself and even then I’d think twice. And I know what to look for, so I consider myself slightly less “vulnerable” to buying stuff that isn’t as good as advertised.
No reputable online site that I know of off-hand that would fit your needs. But perhaps our other readers can chime in here.