The first issue of the Straits Settlements stamps took place on 1 Sept 1867 and the stamps used were basically Indian stamps that were overprinted with the crown and the different rate of postages (See example here) . It comprise a set of 9 stamps featuring 1 1/2 cents, 2 cents, 3 cents, 4 cents, 6 cents, 8 cents, 12 cents, 24 cents and 32 cents.
Of these, I think the 6 cents and 12 cents are pretty rare as I do not seem to be able to find them on eBay. A quick check on some catalogue prices on the internet also seem to suggest that the entire set of 9 used stamps command a price of almost S$2000!
Have been trying desperately to get my hands on this as this is afterall the first issue of Straits Settlements stamps or the first stamps of Singapore when it was formed into the Straits Settlements together with Penang and Malacca. Every collection has to start somewhere and my Singapore collection has to start with this Sept 1867 issue of 9 stamps.
I did a rather quick research on Ebay (based on the final bidding prices or buy it now prices) to check some of the prices of these stamps:
3 cents used = US$54.08 (16 bids)
4 cents used = US$74.26 (20 bids)
8 cents used = US$35.00 (Buy it now price)
24 cents used = US$24.99 to US$40.34 (Buy it now price) OR $67.80 (17 bids)
32 cents used = US58.12 (12 bids)
Hope it is useful reference for all. Apparently, the Ebay prices are going for much lower than the catalogue prices.