It is possible to change the Address and the name on the Checks through the Adsense interface.

What you can not do is change the email address that Adsense sends emails to, thus when then send you something expecting a reply, if it is not your email address you do not get the email and they shut down the account.

As for Legal or not, the question is, do you get full control of the domain that gets the email from Adsense, if you do then buying the account would be fine, but if the code is used on and abused, then you could loose the money when the account is shut down because of abuse, thus you loose the $40 that you purchased for $20.

An interesting Offer, but if it does not come with the domain, then its a looser.