Every registration is required to have an email address or at least, a way to contact the registrant. Normally, the domain will have a for sale display with contact info if it is, in fact, for sale. So, the only prudent way to contact someone using a privacy service, other than that, is through the proxy email address. If they don't respond to an email, I would think that they are not interested.
One might run afoul of cyber-stalking laws if not careful in their persistance.