Hi,
If the person you are selling to has a 1&1-account, then you just have to follow this link:
http://faq.1and1.com/domains/moving_domains_from_one_package_to_another_/4.html
If the person does NOT have a 1&1-account and wants to move it to another registrar, this is the step: (DO NOT jump over nr.1)
1. The buyer pays you.
2. Login to your 1&1-account.
3. Go to "Manage Domains", this is under "Administration.
4. Select the check box next to the domain you want to transfer.
5. Click at "Contact" -> "Edit Administrative Contact" (control if the e-mail is correct), if correct go to step 6.. If not correct it, click "Next", and then click "Submit".
6. For some extensions you need a Authorization code. For example .com, .net or .org., for others you dont need any ex. .cc.
6.1 To get the Auth. Code go back to step 3 and 4, after step 4. jump forward to 6.2.
6.2 Click at "Info"
6.3 Copy the Auth code you see and past it in Notepad (you are going to need this code)
7. Go back to step 3 and 4, after step 4. jump forward to 7.1
7.1 Click at "Transfers" -> "Lock/Unlock" and unlock the domain.
Now it is the buyers turn.
8. Give the buyer the Auth. code, and tell him the domain is ready to transfer.
8.1 The buyer must then log in to his registrar, and follow these steps, (This can very from registrar to registrar but) in generel the steps goes like this:
8.2 After the buyer logs in, he click at a link saying "transfer domain"
8.3 In "transfer domain" page, he has to type in the domainname and the Auth. code you gave him.
8.4 the transfer will begin.
Now back to you, its again your turn. After a couple of days (this differes from registrar to registrar) you have to do these steps:
8.5 You will get an e-mail from the buyers registrar, asking for approval of the transfer, do so by following the instructions in the email.
These steps should do the trick, but it is difference from registrar to registrar. The only step you must NOT jump over is step 1. REMEMBER THIS.
If you have other questions, dont be afraid to ask.
Good luck with your selling and best regards,