In most cases it is "first come first served." Again, the flake to buyer ratio of craigslist is pretty incredible. Anybody willing to hold an item without a deposit is setting themselves up for turning away potential real buyers.
Maybe some of you haven't sold much on craigslist. If you have, and never gotten bullshitted by "buyers" that never show up, you've been incredibly lucky.
In situations where I know someone is driving from more than an hour away, I'll agree to hold an item, but generally under circumstances like - call me when you're ready to make the trip, and I will hold it for a reasonable amount of time for them to make the trip.
For example, if you call me at 4PM and say you're on the way from 2 hours away, I'll gladly hold the item for the rest of that day so you can rest assured it won't be sold out from under you and waste your time/gas.
If someone agrees to sell to you at a certain price and then sells it to someone else that made a higher offer, then yes, I agree totally: that is greed. But that's also human nature, you won't change it, and complaining about it accomplishes zip. Be the guy on the spot with the cash FIRST. That's life.
EDIT - By the way, the subject included eBay though your example just noted a craigslist deal. If someone pulls this shit on you with eBay, you have recourse for it - neg the hell out of them so others will know what to expect. That's why feedback is there.
Richard