The offer and your acceptance are valid for 4 days.
Ebay let the buyer know their offer was accepted and invoice them, giving them 4 days to pay - the same amount of time buyers get after a winning auction bid.
During that time, your item stays live until the buyer pays for it. The sale is not complete until the buyer pays. In the meantime, assuming it is a 'buy it now' item, somebody else could purchase it.
This changed several months ago - I assume Ebay thought it was a good idea.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)