I sold a domain for mid $xx,#*$! in 2004, so I faced the issue of taxation. I am in the U.S. (edit: amusing new auto content filter here... I didn't type #*$ but "x x x". Howard Stern won't be appearing on WebmasterWorld any time soon. :) )
My accountant decided it was a capital gain. I pointed out to him that it's an issue of controversy and that some have expressed the opinion that it is not a capital gain, and taxible as ordinary income. I explained the issue of it being a service contract, and he discussed it with his partner.
Still insisted it is a capital gain.
So, I'd discuss it with your accountant, and make sure that you point out to him that there seem to be varying opinions.
Dunno if this makes a difference, but in my case there was a contract between myself and the buyer, and the terms included all associated intellectual property (software, content, etc.)
BTW, to answer a recurring theme in this forum - the buyer contacted me out of the blue and made an offer that I accepted. No effort had been made to market the domain. The transfer itself was done through an escrow service who seemed to very much know what they were doing, and went flawlessly, save for a couple of grand thrown at lawyers refining the contract.