The definition you found is for one use of "sale":
These items are for sale
WordNet's full item for the quoted definition is specifically referring to sales agreements which is a legal term and explains the smattering of legalize. WordNet also listed this definition which is probably closer to your intended use:
an occasion (usually brief) for buying at specially reduced prices; "they held a sale to reduce their inventory"; "I got some great bargains at their annual sale"
You can sell some thing for a price that has been reduced to nothing. Various stores have sales along the lines of "buy one, get one free." Here the reduction in price is implicit because you are getting an extra product instead of paying less.
But, really, when it comes to marketing it doesn't matter much. The point is to get people to pay attention and download your product. "100% off sale!" isn't bad marketing and you will probably get a few chuckles out of it.