I bought a few watches from carousell in the past few months. There are some really good deals there which I have unearthed, but also have equal shares of scammers. These are the things which I personally do to vet the buyer -
i) If the price is unrealistically cheap, most likely its a fake, I'm talking like 10%-25% of retail price. Use common sense basically.
ii) I check their selling history and read reviews. And I go further by reading the profile of the reviewer. There was this scammer selling an omega aqua terra for around 10k, he left a review using his own accounts. Also use the same profile pic for both accounts, genius

iii) I'm not expert enough to authentic watches, so I insist only on buying watches with full sets, and I will match the serial numbers to the card before I buy.
iv) For anything above 1k, insist on COD, better to lose out on a potentially good deal, rather than being scammed.
v) Trust your gut instinct when communicating with the seller, if he cant answer your questions, or something just feel off, proceed with caution
vi) Beware of stock pics, insist on them sending you actual pic of the watch, do not be afraid of asking questions
Recently I just starting posting on Carousell myself to sell off my own watches too. Now on other end of spectrum where Im being lowballed by dealers who ask 20 or 30% off my price.