People do sell them, yes - but you'll need to look elsewhere to buy them as sales are not permitted here on the official XF site.
There are a number of Admin forums you could try - do a Google search.
Note: You need to be aware, before you buy, that second-user licenses cannot be resold at this time. So if you do buy a used license you won't be able to sell it on at a later date. If you wish to retain the resale value you will need to buy a new license.