This article is from the eBay FAQ, by William B. Shaw email@example.com with numerous contributions by others.
You shouldn't need to wait more than three business days to exchange
contacts with a seller. Remember that eBay states that both parties should
make contact within that timeframe. Unless the seller states in their
auction that they don't use the eBay checkout system, checkout is a great
way to initiate that contact. If the seller doesn't use checkout, you should
expect a timely email from the seller or the auction listing should have
detailed instructions on what to expect.