I don't disagree with you, but that's an issue for the seller, the woman, and eBay to sort out through their dispute resolution process and/or small claims court.
Nothing about her obligations makes lying to her right, just as nothing about breaking the law gives a police officer the right to beat a confession out of a suspect.
Contracts are frequently renegotiated because it's better for both parties than bringing in the big guns. He would rather sell the tickets now than deal with recouping the loss later, and she would rather get the tickets for her money than be forced to pay up for nothing.
Basically, he says, "You can pay now and get the tickets or you can pay up in small claims court after the tickets are expired." Knocking a hundred or two off the price saves him the paperwork involved.
It's still a bad idea to agree to contracts with the intention of renegotiating under the threat that it'll be easier than suing, but that doesn't change the fact that a new contract is easier to make than enforcing the old one.
> Another solution would be for them both to negotiate a lower price.
I disagree. If you want to get something for less than the offered price (i.e. OBO) you should talk to the seller before you place a binding order. Taking a valuable commodity out of play, to force a negotiation, is plain wrong.
Another solution would be for them both to negotiate a lower price.