Never seen anyone pay as much as $500 cash before, normally it is much less.
I am fairly certain they can not be sold mentioned somewhere in the TOS. It would also seem very difficult to change the name and tax identification number on the account without raising red flags. If the buyer did not change the tax id all earnings would be reported with the sellers tax id info and he would be liable for that.
Buying the account would also be quite risky since if Yahoo later realizes it was sold I am sure all money in the account would be lost.