Someone is selling a 2nd hand copy of Hellgate Collector's Ed for only SGD20, which is around USD14. Is it still worth buying at such a low price? I'm mainly looking at it as an RPG-FPS fix, since Borderlands still won't be out for some time. How is the game's shooter mechanics thus far?

Well, the game is shutting down now (or has shut down already), so I recommend you not buy it. It's also kind of crap. I know; I bought the giant, expensive collector's edition.

It is a good game, it has some flaws and some bugs (that sadly will never be fixed :/) but overall a fun experience.
If you get it make sure to get the latest SP patch (should be on Fileplanet - version 1.2 released April 08). Just know that there is no longer a multiplayer feature due to the developer/publisher going under (dang you Roper!).
I have enjoyed playing the game, the MP was fun while it lasted - the SP survives very well on its own. The few bugs are not game breakers at least.
(I have some videos on YouTube about HGL - http://www.youtube.com/carlosjuero)
The only thing that shut down was the MP - SP is still viable.

is MP still available in Korea?

The demo has disgusted me. I wouldn't buy it.

MP is shut down everywhere AFAIK - The studio shut down completely (and Bill Roper moved to head NCSoft West the dullard)

You're best off waiting. Or getting Deus Ex if you want a FPS/RPG fix in the meantime. Now that really is a GOG.
The demo of Hellgate was a real turn off for me. I don't know if it gets better, but with the multiplayer gone, there's little reason to make the investment.

I tried the demo. The Marksman class was bland, very average, so I was wondering if it's changed over the last several months. I'm only interested in the Marksman class.

The demo is pretty well representive of the vast majority of the game. It has a couple of cool missions that are different to the hack and slash/shoot everything thing that moves gameplay but other then those its practically the same throught the entire game. Also the end boss is a right bitch, I walked through the game killing everything with easy up until the boss then he manages to 2shot me, took me about 40 deaths or so to finally bring him down.
Nope hes like that throught the entire game, even his skills are unisipired and all practically the same. Hes the class I completed the game with.

well it was bought by a korean company I believe, with promises that the korean version would get continued updates.

There is currently a Korean company maintained set of multiplayer servers that are still running. They've said that they'd 'like to bring it back to the United States' but Hasbro is being a d--- about it.
I enjoy the game enough, but singleplayer isn't really suffer-able. I'd rather play it in multiplayer if I play it at all, so for now, my disk is stored away.

I'd recommend staying far, far away from this game. Since you've already tried the demo, just imagine much, much more of the same.

Hell no! Avoid it like a mudcrab!

Why would you avoid a mudcrab?