If you included the actual description from the coupon, you really can't be held responsible for someone overbidding.
Unfortunately, buyers don't always see it that way. A few years ago, I sold some stuff for a friend of mine, including some hard to find dolls in two separate auctions. I posted them on eBay UK, listed my location as being in the UK, and eBay listed the auction in UK £.
The winning bidders were in America. One of them paid up, got the doll, and left glowing feedback. The other realised the auction was in pounds not dollars after
the auction finished. She emailed me, complaining, then left negative feedback saying "$45 for this item is ridiculous! Auction should have said it was in pounds!" before I even had a chance to reply to her.
I cancelled her bid, replied to the negative feedback, and sold the doll to the person who'd been bidding against her, who was delighted to get the doll. After that, I made sure to state in big red letters that the auction was in UK£ not US$.
While most buyers are (in my experience) very reasonable, you always get a few who misread then blame the seller.
The best ever though was a woman in America who won an auction, said she had to send the payment via airmail (though I'd specified PayPal only for anyone outside the UK) then sent me an American money order for the wrong amount and made out to the wrong person. She seemed surprised that I couldn't or wouldn't cash it and wouldn't send her the item until she'd paid the right amount.