Generally, the UCC sec 2-201 "statute of frauds" requires that agreements for more than $500 for an exchange of goods must be signed, and in writing. Most states have adopted the UCC or this portion of the UCC.

An informal (unsigned) written agreement can still satisfy this requirement...so long as it represents the intent of both parties to be bound by the contract. But there is correspondence by the domain owner to Jump that he did not throw out the original $50k price expecting it to be accepted or to form a valid contract. And before that correspondence he didn't act like there was a binding agreement.

However, the $100k follow-up email by the domain owner would represent a firm offer that would satisfy the statute of frauds. It doesn't appear the $100k was accepted, and Jump is suing for the $50k price. Ultimately, they will spend a multiple of the original price on legal fees...to try to get the original price. (That assumes they win at court. I would estimate that there is a <1% chance they win if this goes to trial, or to appeals. They chose the wrong hill to fight on.)

Most likely outcome: Jump settles for $100k plus the domain owner's legal fees.