1) Odds are (99.99999%) it would have rang up $19.99, not free. Even the OP notes that it rang up for $19.99, but they got the manager to match the price because the sticker said free. You could show them the image on your phone and make a claim, but I sincerely doubt you would have got the match. The post above mine also notes this.
2) First off, it's only lying if you know that they know the information is incorrect. "to make an untrue statement with intent to deceive" <- Thats what a lie is. Odds are they're just uninformed, inaccurate or unaware of what they're talking about. I do think they should know if they work in the gaming department, but it's wrong to expect that the employees in the gaming department know anything/everything. There are a good number of employees that can't even get their policies right, much less know what features an Xbox has.
3) Intended or not, you were mocking them and are continuing to do so.
A) They did help you, they told you those things don't go on sale, which is generally true. Others have noted that the price rings up for $19.99 as they said. I agree they should have rang it up, though I suppose this relies more on how you asked the question and what you expected them to do. If you just said "are these on sale" and they said "No. They never go on sale", then you're 99.99% in the wrong. If you asked them to ring it up and they said no, then I will agree they're wrong in that instance.
B) The second you say they're lying, you're trying to paint them in a nasty light. You can't prove they're lying, so you shouldn't automatically default on malicious accusations.
C) You can give your reactions and you can give them whatever way you want, but remember that you open yourself up whenever you reply. I think it is wrong to imply you were dealing with low caliber employees, when most of your stance is based off implied information.