The current correct answer is wrong. 판매중 (販賣中, sell+ing) means "for sale" (i.e. being sold), not "on sale" (i.e. discounted).
Yes if it's ON sale we just say 세일
Looks like it's fixed :)
Nope it's still marking it right
I still donʼt understand 중이다. Can someone help?
It's basically another way to form the progressive, like -고 있다
Like ross said "for sale" not "on sale"