In other words, if the buyer told the lady the domain is worth $50k that would be a material misrepresentation?
IMHO most likely not.
Every commercial transaction is premised upon each party believing it is getting a marginally better value.
There is nothing illegal about getting the better end of a bargain.
Arguments seeking to avoid a contract on the basis of "it wasn't a good deal" are the absolute least-favored bottom-of-the-barrel kinds of claims. Courts would be clogged with seller's remorse and buyer's remorse cases.
If you find a long lost Vermeer among the paint-by-numbers canvases at somebody's garage sale, and you buy it for fifty cents, it's yours. Now, that's not exactly what we suspect here, but a claim that even smells like "I didn't get a good deal" when I was in a position to negotiate my terms, is not going to generally be a good ground for a court to re-open a concluded negotiation.