From my experience:

Normally attendees get a conference t-shirt for free. They can tell their size during registration.

Organizers (and their helpers) have the same t-shirts but in a special color, such that attendees can easily spot them when needing assistance.

(I just wore one last night for sleep, but that's another story)

I can't remember witnessing selling off orga t-shirts, but BooK is (in)famous for his huge collection of Perl t-shirts, so he would certainly buy any new one.