I am a Singaporean and have been buying 2nd hand watches from dealers quite regularly. The dealers I have bought from are :
i) JWWatch - located below Great Eastern building near Raffles Place. Their watches are often top quality but the owner is pretty sticky about prices. Prices are not the lowest and non bargainable. Not even the GST, but this should not be an issue with tourist. (
http://www.jwwatch.com/)
ii) Hobbylink - located at Far East Shopping Center. Owner Sam is easygoing but is not a certified horology technician, unlike John from JW. Sam's prices are negotiable since they are mostly consignment pieces. Sam's main business is in remote control cars so his watch is stored in a simple and non attractive display. I was pretty surprised that his simple cupboard has so many expensive watches. Pretty suspicious initially, but after one deal, I am convinced. Note that his watches are often in original condition.
(
http://www.hobbylink.com.sg/Watches.htm)
iii) Koh Watches - located at level 2 People's Park Centre. This shop is my personal favourite for 2nd hand Rolex. Prices are good and service is friendly. This is a family run business with another shop named AK located in the same floor. They only deal in Rolex, but their pieces are mostly without box and paper. Although I prefer them because of price, I would buy from JW if the prices are the same. No website. Aunty owner mentioned that business is too good, so no need for website. Since no website, do go there if you have a particular model in mind. Also do some research about the price and series before hand.
iv) Bronze Age - located at level 2 Chinatown Point, which is across People's Park linked by an overhead bridge. Owner is quite reserve but he wil give good discount off the published price if paying by cash. (
http://www.ebronzeage.com/)
The other shops have been mentioned by fellow repgeeks e.g. Passions, Visio time and Top Hour. Personally, I did not buy from them mainly because their service is lousy despite non attractive prices. Just my view, maybe they treat their regulars better. Anyway, many options around so normally I will not waste my time with people who are not interested in my hard earned $$.