Demaestro - 5:25 pm on May 5, 2010 (gmt 0)
A domain that might have been worth $750-$1500 could eventually (2 years later?) become a $200K domain by virtue of circumstances that no one reasonably anticipated at the time of the initial transaction.
Even more to the point, the domain could have been developed, gained a large user base, and it was the traffic, the site, and the domain that was purchased for $200,000 in which case you would have ZERO claim to the big sale money because the domain was developed and it's value raised and had value added by work not done by you.
They could even say they bought the domain for $700 and they bought the site and traffic for $199,300