Digital Point is the best place to sell for free, but you will not get much out of a vB license now days due to the licensing changes they made and the fact that you must buy a new license with every major release.
I would not buy them, sorry but that is just an honest opinion and I own 3 "vB 3 lifetime licenses" which have little value due to IB destroying their value.
There's a number of people asking for vbulletin licences over at http://www.theadminzone.com/forums/index.php Might be worth your while ducking in there and offering them for sale. I would get in quick though as certain undesirables are posting up Xen in a big way.
I have an owned license, but I'm not even gonna bother trying to sell it. Too many people now are looking for xF so I doubt I'll get anything worth worrying about for it. Not only that but mine is only for the vB3 license, which is arguably better than a vB4 but still it's old hat!! I'd rather hold on to mine, see what happens in a few years and if the market is with vB again, sell it then!!
Another place to keep an eye on is eBay. They seem to be having quite a few free listing days, so that could be an option!!