I never used to put firm in my craigslist ads, but after selling a bunch of games I've learned this:

a) if you don't put firm, you're always going to get offered lower. So either you put the game at $2kobo to get $1800 or just list it at $1800 and stop the charade. I prefer the $1800 approach.
b) firm keeps away people just wasting your time
c) if someone responds to an ad in all caps or bad grammar, don't respond. Not worth it.

And I'm not charging ridiculous amounts for games, unless you think $1650 for a Goldeneye or $2400 for a Corvette is unfair.

It kind of depends. For some people (like me) you've barely done anything to make me want to pay more. Sound system, yes, everything else, meh. I basically want to put the game in my basement to play (doesn't have to be A+ restored by any means), and I'd rather have a $2k congo that's easy to sell when I'm done then a $3k to $4k congo that's hard to sell. Plus I really don't want to spend $3k on a congo to begin with, if I'm getting into that area I might as well buy Whitewater or some other $3k to $4k range game. That's just my opinion though, I'm sure most people don't share it.