Disregarding all the usual complaining each way...
If the item says it's only available for a Limited time, by UK law there's actually regulation that prevents you then selling it generally afterward as misrepresentation of sale in order to encourage sales, and I quote:
"likely to materially distort the economic behaviour of the average consumer in regards to the product."
So for those saying 'it's fair game, it'll make Frontier more money, which means more expansions' - No, not if it gets totally legitimate legal action taken against them in response.
Personally, I couldn't care less - But Frontier, despite their size, need to be aware others might, and if they have thrown around binding terms like 'Limited Time Offer', manipulating these can have serious repercussions.