An example, as of now: BTC is trading at (my country Singapore) S$23,900 but on localbitcoins, sellers are quoting S$26,000 plus.
Why so? I do understand that there is a TX fee and some profit margin for the seller, but S$3000+?
I might as well buy from coinbase and sell BTC at localbitcoins to make a profit..??
There is no such thing like actual BTC price. The price of Bitcoin is shaped from its demand.
The international price is the average of prices of markets around the world. The demand of bitcoin is higher in some countries than the demand in other countries. So the average price is Combined Arithmetic Mean of these prices. While what you see on LocalBitcoin is the real price at your location. If it is higher than international price then it means demand in your location is higher than worldwide average and people there are ready to pay higher prices. This is nothing to do with TX id or other expenses.
PS: If you see such difference, you can make good profits using Arbitrage Trading. Search Google for more info.