Sales have become so common that when you walk into a store and see a sign touting 40 percent off, you might be dubious. Turns out you’d have good reason to not believe the reduced prices.

The problem, says Jack Abelson, a retail expert in San Jose, California, is that retailers are offering merchandise labeled as being on sale that they never sold at a higher price. When that happens, they are deceiving consumers. The Federal Trade Commission says that retailers should make items available at “list price on a regular basis for a reasonably substantial period of time” before describing it as being on sale.

Consumers Fight Back

Consumers, wise to these slippery sales tactics, are increasingly fighting back. This month, Nike was hit with a $5 million class action that charges the company with using misleading price tags in its outlet stores. The complaint says the price tags offered discounts off of a false “suggested retail price” that could be confused as the manufacturer’s suggested retail price, or MSRP. According to the complaint, within the three months preceding the “sale,” Nike Outlet Products did not sell the products at the “suggested retail price” or even at the manufacturers suggested retail price as required by California law. Nike did not respond to our request for a comment.

The Nike case is just the tip of the iceberg. Last year, retailer Michael Kors was hit with a similar class action that accused the retailer of misleading consumers between 2010 and 2014. The lawsuit claimed that the retailer used price tags that led shoppers to believe they were getting greater markdowns than they really were at its outlet stores. The case was settled in federal court in New York. Michael Kors, which denied wrongdoing, agreed to establish a $4.8 million settlement pool (at least 70 percent of which will go to consumers, with the rest earmarked for legal fees). It also agreed to change some of its outlet pricing practices. We reached out to Michael Kors for comment. A spokesperson emailed us to say the company does not comment on litigation.

Earlier this month, a California woman and 100 other consumers filed suit in the state's Superior Court, accusing the jeans retailer, Guess, of running a fake sale. The suit claims that discounts offered at its factory outlet stores were based on fabricated list prices, a violation of California law. We reached out to Guess for a comment but a spokesperson emailed us to say the company did not wish to comment.

For such illegal sales to stop, the regulations that govern sale prices will have to be improved—a slow process since pricing is regulated at the state level. Meanwhile, though, all the bad publicity and penalties could cause merchants to tone down their marketing tactics, says Vicki G. Morwitz, a marketing professor at New York University’s Stern School of Business.

For now, shoppers should not let big sales touted by retailers sway their good judgment. The FTC also advises that you keep an eye out for "creative" math on store tags and weekly ads that can be deceptive. Be sure to also watch out for “liquidation sales” designed to make you believe prices are dramatically marked down. Sometimes, at the beginning of a liquidation sale, a retailer may charge full price until inventory clears out.

How to Get the Best Prices

If you aren’t sure of how good a sale is, use an online pricing-comparison tool to see how much other retailers are charging for the same product. Some to consider:

PriceGrabber lets you compare prices on goods from thousands of merchants and sellers. You can also read merchant ratings and reviews, pour over detailed product information, and get email notifications of the best available prices.

Savings Catcher, created by Walmart, allows you compare Walmart’s prices to those of other retailers, including Amazon and Target. You can also download the Walmart app, tap Savings Catcher, and scan your receipt. The tool then compares the price of the items on your receipt to the advertised prices at other stores in your area. If it finds a lower price, Walmart will refund the difference on an e-gift card.

Shop Savvy tracks items you place on a list and notifies you when they’re available at a discount. A new feature lets you check prices via Twitter. Just Tweet a link to a product you’re considering to @shopsavvy and you'll get a response if it's available elsewhere for less.

ShopAdvisor is an app that lets you track products and prices at more than 16,000 online and in-store merchants. Besides alerting you to sales, the app analyzes sales data to inform you when an item you want is likely to be available at the best possible price.