There's no easy answer to this question that comes to mind, unfortunately, but it is interesting that unsolicited faxes are illegal yet unsolicited calls & mailing are lawful. It looks like you'll have to try other contact options.
Out of 80+ impending end-user sales on which the buyer & I have agreed on the terms over the past few months -- including one $1800 agreement which would have amounted to my most colossal sale to date -- around 5 of these end-users mysteriously "disappeared" after that point. Another 5-or-so other end-users changed their mind and reneged on the impending sale, often without stating why. It happens often (roughly 10-20% of he time), and it hurts when it does.
But many times a potential buyer will only "disappear" for a week, then come back ramifying they needed to 'put out a few fires' or 'deal with a sick parent' before re-focusing on business development. If you're out actively chasing end-users, you're probably more eager to sell (and more time-pressured) than the prospective end-user is to buy.
In your circumstance, I would start by sending a gentle-sounding e-mail to your end-user with soft deadline:
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Hi John,
I just wanted to check in on whether you feel ready to make the XYZ.com swap we agreed on a few days back. I'll keep our agreement open through Thursday evening unless you feel a later date would be more convenient.
Hoping all is well,
Joshua
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In your e-mail, allow them about 3-5 business days to respond. If you don't hear from them by roughly 24 hours before your deadline, call them and say something resembling "I wanted to call because I haven't heard from you in a while regarding our XYZ.com agreement and was just wondering whether you're okay." Play it cool, and be sure to inquire (at first) about their well-being rather than the domain. This strategy has worked well for me lately. Coming on as a compassionate person might make your end-user feel as if they're giving to a good cause and hence ease their hesitation.