How does a seller send an invoice thru ebay for a coin not listed for sale?

I have made several purchases in the past by making offers for coins that had expired unsold and the seller immediately sent me an ebay invoice for the coin(s) all thru ebay messaging and I paid from that invoice. That has happened to me and I always just relisted the coin for the agreed on amount, but I have wondered how did the sellers send me an invoice without listing the coin?
Jim
When a man who is honestly mistaken hears the truth, he will either quit being mistaken or cease to be honest....Abraham Lincoln
Patriotism is supporting your country all the time, and your government when it deserves it.....Mark Twain
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eBay put a "make an offer" button into the messaging system.
"It's like God, Family, Country, except Sticker, Plastic, Coin."
I suspect the seller relisted it. I know of no way you can send an offer on a coin not listed on ebay.