It's actually the same problem, as the seller's copy is just a Bcc: on the message we send the buyer, so both copies are one and the same message.
However, our sysadmin has not found any anomalies with outbound mail, and free links I've sent myself to test this morning have arrived within seconds, so we can only conclude there may issues on the receiving end at some email provider(s).
By any chance do all of the affected buyers have email addresses
@hotmail.com/outlook.com/msn.com/live.com/passport.com, or perhaps custom-looking domains that might redirect to those? The ones we've looked at so far all seem to be one of those, which would seem to indicate some issue on Microsoft's webmail services today.