Quoted from fosaisu:

IMO it's not a dick move to offer more than list price for an under-priced item. He's just increasing his chances of making a buy.
Seller is under no obligation to accept the first email saying "I'll take it". If she made a commitment to sell to someone at asking price already, it's on her to honor that commitment and if she doesn't then that would-be buyer has a right to be pissed (but not surprised - this is Craigslist, not Pinside).
If he's actively trying to talk her into breaking her word to someone else I'll agree that's lame. But if she says "I got an earlier email from a guy that'll pay the $400" but doesn't say that they've solidified a deal, and he says "I'll pay $1000" and reminds her that Craigslist doesn't require you to sell to the first person that emails, I don't see a problem there.

Quoted from NPO:

I'll politely disagree here. I guess this is where some people just feel differently.
To me, if I know someone else has already been committed the game, and I know at that point that me offering more money is, in essence, me trying to take the game away from someone else who has already been committed the game - that's plain thievery.
I'll bet if you offered more money and won the game, and you just so happened to meet the previous buyer or the previous buyer was one of your good friends who you took the game from, you sure wouldn't say anything.
For me, when someone has been committed the game, I back off. I ask the seller to keep my name at the ready in case the first buyer flakes, and then it's my turn. Everyone else's MMV.

Quoted from CaptainNeo:

If nobody had offered anything yet, and then offered more to secure it, that's a different story.