If Jud offered the Krauses considerably less for their property than its possible market value and the Krauses accepted the offer, could they void the contract on later learning that they might have sold it for a higher price?
With any legal question, the specific details of the situation matter immensely. However, if we just look at the basic premise of this question, there is no reason why the Krauses should be able to void the contract. It appears that they simply made a bad deal and will have to live with that deal.
A contract can be voided if one party induces the other to sign the contract through misrepresentation or fraud. So, for example, if Jud had lied to get the Krauses to agree to the lower price, the Krauses could get out of the contract. Let us imagine that Jud falsely told the Krauses that a slaughterhouse was going to be built next to their property. The Krauses would then sell to him at a low price because they would know that their property was going to lose value once the slaughterhouse was in place. The Krauses would then be able to say that they had been tricked into giving the lower price and they could void the contract.
However, there is no evidence from your question to suggest that Jud made any fraudulent statements. The question implies that the Krauses simply accepted a poor offer for some unknown reason. Unless they have some sort of clause in the contract that allows them to get out of it if they receive a better offer, they are stuck with the contract they have signed.