I dunno, people usually pay to get the character leveled
faster, rather than
cheaper. It might be a good deal if you want to save some money, though.
I found something in the end user license agreement a leveling site might find interesting, so here you go.
Section 3B of http://www.worldofwarcraft.com/legal/eula.html :
"B. You may permanently transfer all of your rights and obligations under the License Agreement to another by physically transferring the original media (e.g., the CD-ROM or DVD you purchased), all original packaging, and all Manuals or other documentation distributed with the Game; provided, however, that you permanently delete all copies and installations of the Game in your possession or control, and that the recipient agrees to the terms of this License Agreement. The transferor (i.e., you), and not Blizzard, agrees to be solely responsible for any taxes, fees, charges, duties, withholdings, assessments, and the like, together with any interest, penalties, and additions imposed in connection with such transfer."
So I guess basically if you do the direct online upgrade, you have no physical media, so you could "legally" transfer the account to someone else's ownership. The end kind of gives them a loophole though: they can ban you if they damn well wish to.