'on sell' is wrong. 'on sale' is not past tense of 'on sell'. 'Sale' is a noun, a thing, not an action. 'On Sale', 'discounted' or 'marked down' are all terms related to retail sales. Hope this helps! :)

"On sale" is correct. "Sell" is a verb, "sale" is a noun and sometimes an adjective.

"On sale" is correct in my part of the United States (New England) and means the items) is being sold at a discounted price. We also say "for sale" which means an item is being sold at a particular store. For instance, "They have several brands of tents for sale at Walmart in Hadley. I ended up buying a Coleman tent which sleeps 10 people. You should see if they have one you like and could use for your camping trip."