From my admittedly limited experience of buying 1 watch from chrono24 (a dealer in Poland to myself in the UK) here's my take on things.
1) Due diligence is all. Find the watch and dealer you want to use and then dig into them - ask here if anyone has used them (i got a positive review on here from another member), look into their web presence (if any). Ask for more photos of the piece focusing on a particular part (i wanted to check the condition of the leather strap and buckle on my purchase).
2) More Due diligence. Find out everything about the watch you want to buy. Know all the details, what paperwork should be available (yes it's still worth having). What box it should come in (again worth having).
3) Even more due diligence. Take a dive into the murky waters of fake watch forums and see if your prospective purchase is known for being faked and especially if there are 1:1 or super clones available. Certain Rolex and Breitling models are scarily close to the originals - and personally I'd only be willing to buy one face to face from a bricks and mortar seller with business insurance so I could get my money back if it was discovered to be a fake.
C24's escrow service is worth using as they only release your money to the dealer once the goods are received and signed off by you.
Haggling - C24 will discourage you from making an offer even when the option is given by the dealer. I knocked £200 off the price and this was flagged as 'a significant drop in price which will in all likelihood be rejected by the vendor who will then track you down and eviscerate you for the affront offered' or words to that effect. I'd already agreed a price over e-mail which is probably the best way to do it. As long as the agreed price is actioned through C24 you are protected.
Finally general consumer advice - use a Credit card if possible as it affords you extra protection - but you probably knew that already.