Quite clearly, if the property is legally fully developed, there is no way out of the contract. Even if it were not fully developed, that does not invalidate the contract; rather, in that case, there would have to be deliberate deception. The only option you have left is to try to negotiate the purchase price with the seller, but at the moment I see no reason for that.
The fact that you personally know the seller is of no use. Of course, it is annoying for you, as you could have also obtained the property that way, but you did not. The question is rather why you did not notice the Ebay Classifieds ad. Consider it a lesson learned, especially since the situation with just land is different from that with an existing property.
A little info from my own circle of acquaintances:
My best friend also saw a property online through an agent and asked the agent to send the exposé. On the same day, his wife heard about the property through relatives. When my friend realized in the evening that it was the same property, he immediately canceled the inquiry with the agent, but since by having the exposé sent he had already waived his 2-week objection period, he ultimately could not back out. Note that no viewing took place through the agent and no purchase intention contract was signed. In the end, the seller bought out of the agency contract (which was somewhat cheaper than the commission), which my friend then took over.
Of course, one gets annoyed by things like this, but ultimately you actually found the property through the agent, and thus he earned his money both legally and morally, period! What you are now trying to do is breach the contract, and I would be very careful with that.
PS: I also believe that some agents are paid too much for the service provided (but only some); furthermore, I do not understand why the purchaser-pays principle does not also apply in the purchase, since the seller clearly benefits more from the agent than the buyer. Nevertheless, this is the current legal situation and you know this beforehand. If I am not willing to pay a commission, then I do not search through an agent. Plain and simple.