Have they paid?
Once they have paid through Ebay checkout, you should be able to exchange details.
There is sometimes a bit of delay between the buyer paying and the Ebay system allowing you to email with details though.
"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)