I don't think it is always a black and white answer....or that one has to stand by the same policy in every situation (though some consistency certainly helps if an unhappy rejected buyer decides to complain).
A hobby breeder has a lot of flexibility with this decision, and can wait a couple weeks for payment if they choose...it's as simple as pulling the ad, and shrugging off the annoyance of people not completing the deal. What if said flexible hobbyist finds himself in need of money? First contact policy out the window, now it's first pay wins.
What if it's a brick and mortar shop that posted ads online? Face to face deal with a paying customer...or wait for the guy that has sent 3-4 emails? It's pretty hard to turn down the person standing there that wants to buy; and what if the seller does say, I'm sorry, but it's sold. It isn't necessarily a given that other interested parties will purchase after they have been turned away...and they have overhead that the hobbyist doesn't need to consider (at least to the same extent).
As a seller, I have exchanged multiple emails with prospective buyers, and declined sales while waiting for payment that has never arrived. I have also given up on prospective buyers and sold an animal...only to have the first person try to purchase after I've taken payment from somebody else. It's part of selling; and all we can do is try to handle these situations in the most professional manner possible.
As a buyer, I have committed to a purchase & been waiting for payment information that never came. When I contacted the seller the following day, I was told that I failed to make payment so he sold the snake. Yes, I was upset...and have not considered a purchase from that seller since. I understand his position.....but how was I supposed to pay when he didn't provide me with the means? Conversely, I have also committed to purchases by email, and been unable to make the phone call to pay by credit card for a few days (I hate phones, and am rarely available to make/receive calls during some people's normal business hours) Sometimes the sellers had waited, sometimes they hadn't. Business is business, no hard feelings there; but I definitely appreciated the ones that had waited for me (in most of those cases, there was an existing relationship...and they knew that I would complete the transaction).
But let's add a twist to this.
Is a seller required to sell to the first person that makes contact, or the first person that pays? What if a person sends an email, and immediately follows with a payment; so the seller comes online to find both...but doesn't want to sell to that buyer? Anybody have a problem with that (aside from the scorned buyer, that is)?