So how could I possibly have done anything about this within the timeframe?
By opening a case under eBay's money back guarantee.
Being "out of office" would have meant nothing. The seller would have been required to respond within three working days. If they didn't, you could have escalated the case for eBay to step in and they would have enforced a full refund.
Buying from unknown sellers on eBay isn't like the shops. The user agreement warned you about the risks, that "eBay does not guarantee the existence, quality, safety or legality of items advertised, or the truth or accuracy of descriptions".
The onus is on you to read and understand how eBay buyer protection works, and use it. It's too late to do anything through eBay now, but if you paid by a credit or debit card your card issuer may still be able to help.