From the looks of the who is details, it is registered with a company called MonkeyBright.com which actually looks like a front end reseller for Register.com.
The help section on MonkeyBright is actually registers and the following info comes straight from there.
How do I transfer my domain name to another Registrar?
Question
How do I transfer my domain name to another Registrar?
Answer
We'd rather not see you go, but if you'd like to transfer your domain name from Register.com to another Registrar, you'll have to contact the other Registrar to begin the transfer process. We are unable to initiate this process for you.
You should be aware that each Registrar has a unique process and timetable for transferring Web addresses. In most cases, you will be required to confirm your transfer request before it is processed. In addition, there may be reasons why your Web address cannot be transferred. If you initiate a transfer close to the expiration date and the transfer is not successful, you will lose your Web address if it is not renewed with us.
If you do choose to switch to another Registrar, please be aware that any Register.com services you are using -- including email, FirstStepSiteยฎ, Web Site Forwarding and WebSiteNOW!โข -- will be terminated upon successful completion of the transfer away. We are not accountable for disruptions to service caused by a Transfer of Registrar.
Quick summary means that the person you sold it to has to initiate the transfer from the register they want it to end up in - this will cost them the price of a renewal year. Register has a 200 USD processing charge to transfer it for you so the cheapest and best option IMO is to get the new owner to initiate the transfer from his end. Best checking into all the small print at register first though rather than simply taking my word for it.
Good luck with your transfer ckyle. Also, next time you want to register a domain - shop around as there are way better prices than register.com's $35 for a .com. I and many others on the board can do you a domain for $7.95 if you want and/or set you up with a free $8.95 account at enom.
Anyway, congratulations on your sale