E-mails getting lost in the ethers is not uncommon. They also get filtered to spam on occasion. I've learned the hard way in my line of work to check the spam folder daily, as sometimes important messages find their way in there.
Really, one shouldn't go complaining in public before trying at least two methods of contact (
) - even if the second is just a second e-mail. However, I always worry about "pestering" the seller or being a nuisance, but in light of the above, a second e-mail a couple of days later or so really isn't overkill.