During the sale
People who want to buy your item follow your link to the sale page and sign up with their name and email address, or they can connect with Facebook and their details will be pre-filled in. On entering this data they are presented with their entry number.
Optionally when you setup the sale you can allow people to give their entry to others (In which case in addition to their own name and email they can provide the name and email of the person they wish to enter for).
A confirmation email is sent to the entrant with their entry number. If they entered for a friend then a confirmation message is sent to them as well.