For the same reason I'd avoid eBay or Craigslist: plenty of legitimate sellers with listings on all those sites, but also a lot of scams, and their recourse isn't the best. I've bought from eBay and C24 without issues; I've also had serious problems with both sites---thankfully not huge dollar amounts. Anyone can list on those sites--the vetting is poor. A major trusted seller, like the ones I listed: you know you'll be ok---they authenticate every watch, and they live and die by their reputations for selling legitimate, quality watches. If they make a mistake, they're going to resolve it. You might end up paying slightly more for this peace of mind, but I think it's worth it.
Another thing about C24---many
many of their listings are bogus, for some references nearly or actually all of them are. It's there in the fine print, which you
have to read (I'd say) before clicking buy: many listings will say things like "Item needs to be procured" (
here's one, for example, literally the first hit I got when I visited their site). So you click buy, you submit payment, and then your money is tied up and you have no watch until the seller manages to find one for you, if they ever do. Maybe it's not technically a scam, but it sure seems dishonest to me. If you're going to roll the dice on C24, be sure the listing says "Item is in stock" and they include several real photos of an actual watch in their possession. That will increase the odds of you actually getting a watch.