I do not think this was unethical. Annoying and poor customer service, perhaps, but certainly nothing that would warrant contacting the Better Business Bureau like Vidusa suggests.
Winston, perhaps you could explain something to me. On your list of Saturdays' events, between working out a mutually acceptable deal in Step 2 and later returning to actually purchase the animal in step 3, what transpired? Were you one of those numerous customers who showed interest, asked a few question, and then said you wanted to think about it or look around more? If so, then I don't think any deal was ever really settled. If he made an offer for a discount price and even ignored a red-flag like a post-dated check and you failed to accept and pay up, then there is nothing unethical about not jumping at the same deal the next time you show interest, especially if he has people showing interest at the asking price. It may not be a great business practice to turn down a paying customer now for a potential customer later, but it isn't unethical.
If you waffled about the deal and didn't pony up, then he's free to make other deals for his animals. From what you've posted, the seller never agreed to hold the animal for you. As far as I am concerned, that animal was fair game for any of those other customers, especially if they were topping the amount you offered to pay.
Again, I think it is a somewhat dumb way of dealing with a customer. However, I don't think it is unethical to follow through on a scheduled meeting with another customer who is willing to pay more rather than offer the animal again to someone who's already effectively passed on one discounted transaction. That is my take on Saturday, step 5.
Along the way, you say you continued to be agreeable about all of this, and never offered to match the other customer's offers.
Nice of you, but you were leaving the doorway open, showing the seller you'd rather risk losing the animal than pay asking price.
Sunday you again try to pick up the animal while they have another customer who is coming to pay (presumably full price) after church. The way you word this is different than the other times, so I can only conclude from what you say the seller told you that this was not a customer thinking about it, this was a customer he'd worked out an agreement with already. Someone who accepted the offered deal, and got an agreement to hold the animal, most likely. Not someone who was offered a deal and decided to look around or think, like customer A and yourself?
In the meantime, they get another animal you want, but you tell them to sell that new animal to the person they've arranged a deal with.
You insist that there were four different customers, but since you never saw customers B and C, don't you think it is pretty possible that B, C and D are all the same customer? That he went and met the customer that night to show the animal, and they made an agreement that the seller would hold the animal and the customer would pick up and pay the next day after church? Not impossible, at least.
So, in effect, when you try to talk him in to selling you 'the little one' and giving the other customer the other one (that you also wanted), you were asking him to pull a bait and switch?
And you say you waited until he told you he had a deal worked out Sunday for the animal before ever offering him his asking price. Did you bother to offer him his asking price on the other one that morning? Did it never occur to you before if you wanted it all that terribly badly to agree to the asking price the previous day, after you'd more or less passed on a lower offer?
At that point, they refuse your business (or at least your check). Granted, I do think it is possible they refused it for no better reason than that you had annoyed the living poo out of them, but that is not unethical. It is their right as a seller, especially after you were the one who walked away from the first quasi-deal you'd hammered out.
Sorry for your wasted time and annoyance, but I don't think anything unethical happened. He wasn't under any obligation to sell it to you, after all. A customer with a handful of cash (or a post-dated check) doesn't mean you have to complete a transaction. A little bit of negotiation is not a verbal contract. It does suck that you were strung along a bit just to hedge the sellers' bets, though.