Looking for deals on Black Friday? Watch out for this marketing magic

If you're looking for Black Friday deals this week, sales will be more competitive than ever as retailers move prices around to get rid of built-up inventory, one expert says.

With some offering flat dollar amounts off a price, and others pushing percentages off, retailers make it hard to compare the best deals across stores, Mike Bernacchi, professor of business administration at the University of Detroit Mercy, told CNBC's "Power Lunch" on Wednesday.

Take, for instance, confusing "stacked promotions," where an additional percentage is lopped of an item that has already been marked down, Bernacchi said. Or the trick of where an item is marked with a steeper discount at one outlet, but has a higher original price than other stores.

With the pressure to beat the Black Friday crowds, it's tempting to skip the math and make a knee-jerk decision — and miss the best deals in the process, Bernacchi said.

"I'll tell you what happens: you almost buy on instinct," he said. "You buy like it's an impulse purchase and that's what the retailers want."

Bernacchi said not to wait until you hit the stores on Black Friday to do your homework.

"The world is for those who are prepared," Bernacchi said. "I promise you retailers are prepared, so we better be prepared as well."