It's been years since I was there, but the shopping mall MBK used to be a good place to peruse.
Plenty of grey/second hand dealers to be found. Not sure about pricing given how weak the pound is, but some good deals to be had if you know what to look for. Bangkok used to be a mekka for second hand watches many years ago. Not sure how things are today.
Fakes are mostly for tourists and sold on the street. A reputable grey dealer in Bangkok or Hong Kong for that matter wouldn't be caught dead with one. So would advise easing off on the stereotype when you are there, to save you a good beating