I have recently purchased Valve's Orange Box which includes HL2, HL2:E1, HL2:E2, TF2 and Portal.
I already have Half Life 2 (bought when it originally came out) and as such, when I've typed my CD key into Steam, it has prompted me that I already own HL2, and have the option of transferring the Licence for Half Life 2 to another person.
If anyone wants to transfer £5 into my paypal account, give me a shout, and I'll put your email into Steam to transfer the licence.
You don't need any previous versions of Half Life for this, the email that Steam will send will expain everything you need to do (Install Steam if you don't already have it)
Half Life 2 is an excellent game, and well worth £5!
eBay Feedback under the username: AdamBazUK
Email will be sent as soon as payment is recieved.