Here's what eBay says about it:

"eBay prohibits email offers to buy or sell listed items outside of the eBay site. Offers of this nature circumvent eBay's fee structure and are a potential fraud risk for both buyers and sellers. Some examples of off-site offers include:

Using information obtained through eBay to offer to buy or sell a listed item off-site.
Canceling a listing to sell to a buyer who became aware of the item through eBay.
Ending a listing early to sell the item at a higher price to the winning bidder.
Offering to sell an item to a bidder in a Reserve Not Met listing.
Offering to sell duplicate or additional merchandise to underbidders.
It is acceptable for sellers to end a listing early in order to sell an item at the current bid price to the high bidder. Bidders are permitted to contact sellers with requests to end a listing early; however, sellers are under no obligation to do so.

Although off-site offers are not permitted, sellers may be able to sell items to underbidders by sending a Second Chance Offer."