More than 90% of Black Friday deals were the same price or cheaper in the six months before the sales event last year, a Which? investigation has found.
The consumer association told shoppers to make sure the discounts were "truly genuine" ahead of this year's sales.
It said 184 out of 201 items from six retailers, which included Amazon and John Lewis, were priced the same or cheaper before Black Friday in 2020.
Trading standards warned a deal "may not be what it's all cracked up to be".
Katherine Hart, lead officer at the Chartered Trading Standards Institute, added the "prospect of scams rears its ugly head" during the sales event, which takes place on Friday.
"It seems that the Black Friday and Cyber Monday sales events get bigger every year, and many bargains can be found; however, we all must remain vigilant and practise shopping safety," she added.
"Sometimes sellers raise their prices before a sales period to make it look like a great deal is on offer during the sale."
Which? said popular items it found to be the same price or less before Black Friday last year included washing machines, soundbars and TVs.
The 'worst deals'
It analysed items from Amazon, AO, Argos, Currys, John Lewis and Richer Sounds by looking at the prices offered by the companies every day during the six months before and after 2020's Black Friday, as well as on the day itself.
"When looking at prices six months before and after Black Friday 2020, just one of the 201 products was at its cheapest price on Black Friday alone," the consumer champion said.
One of the "worst individual deals", it added, was a Zanussi washing machine at John Lewis that was cheaper than its Black Friday price on 88 different days before the day itself.
"'Discounted' to £309 on Black Friday, customers could have bought it £60 cheaper at £249 five months before and for £289 within just a month after," Which? said.
Richer Sounds had the highest proportion of deals with four out of 14, that were not cheaper or the same price before Black Friday.
'Do your research'
Ele Clark, Which? retail editor, said it was important shoppers did research before diving into the sales.
"Take time to identify the products you really want and check that the 'deal' you're seeing represents a genuine saving," she said.
In response to Which?'s investigation, Amazon said it sought to "offer our customers great value thanks to low prices all year round" and that customers could "easily compare prices, allowing them to make an informed purchase decision".
Meanwhile, John Lewis said it monitored prices all year round and match high street competitors as part of its "never knowingly undersold" policy.
It explained a portable music speaker advertised on last year's Black Friday was the same price between 20 November 2020 to 12 January 2021 as "those days were actually all part of the Black Friday promotional activity".
Richer Sounds admitted a Yamaha music speaker, which was priced in last year's promotion at £159 with an RRP of £199, had actually been that price or less on 277 other days and hadn't been for sale at its full price at the store since 2018.
Richer Sounds said the deal was a "mistake" and apologised to any customers who were misled by it.