A: There’s a big sale at the shopping site. Check it out!

B: Look carefully before you actually buy anything. Sometimes these sales promotions are nothing but smoke and mirrors. Some online shops would mark up the prices before they offer you an attractive discount.

Note: Smoke and mirrors are words and actions that are intended to deceive or confuse people, especially by making something seem better than it really is. The idiom can be used attributively when hyphenated. The usage alludes to magicians using props like smoke and mirrors during their performances.