They're tradable;
I've always suspected shiny prices are driven up mostly by people who buy plat from websites etc., since it's effectively a way to quickly convert pp to exp/AA. But I have to confess if I want to quickly level a new character, a bunch of level 60+ collections can work wonders. Perhaps these collections are bought by powerlevelling services too, since it's presumably easier for them to farm plat then buy every collection and ding 0-50 in a few hours than actually grind the exp.
I've never been sure, really, if collections giving ridiculous exp at lower levels is such a great design idea, but it's definitely a popular one.