If the domain is a .com or a .net I don't think you should have too much trouble. I transfered a .com and a .net from 1and1 to domainsite last week and it went very smoothly. Initiate the transfer from the registrar that will be acquiring the name and don't forget to unlock your domain at 1and1.
If it's not a .com or .net then you will need to contact 1and1 for an authorization code. Dealing with their customer service may prove frustrating. I haven't had to do it yet but the stories I hear make me fear that day.
Never reg a domain at 1and1 again. Shocking customer service in my view. I tried to transfer a .co.uk to a buyer but i couldn't making me loose a healthy profit....anyway.....hiphip.co.uk available to buy again!
if you are even 1.1% serious about the selling of domain names then do not use 1+1 to register your portfolio unless you think that pissing on your own shoes is a good way to keep them clean, in which case go ahead