The buyer has 4 days to pay after they accept your offer.
There is not really any need to contact them.
Ebay have changed how offers work quite recently and now, even though you have accepted an offer, the item will remain for sale until the buyer actually pays.
This is in the seller's favour as you may get a higher price.
However, if buyers are slow to pay, they can lose out.
"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)