Nearly nine in 10 products sold on Black Friday are available for the same price or cheaper earlier in the year, according to consumer group Which?.
It urged consumers to “do some research” in order to spot genuine bargains during the 27 November sale.
Many shoppers see Black Friday as a way to buy gifts cheaply before Christmas.
More than two-thirds have delayed a purchase to try to find a bargain in the sale, according to Lloyds Bank.
It expects Black Friday spending in the UK to soar to £750m this year.
Which? tracked the prices of 219 popular home and tech products for six months before the 2019 sale and for six months afterwards.
It looked at goods from Amazon, AO.com, Argos, Currys PC World and John Lewis and found only three items were at their cheapest on Black Friday.
Natalie Hitchins, head of home products and services at Which?, said: “Deals that look too good to be true often are, so don’t fall for time-limited offers and if you are looking for something in particular, do some research first.
“That way you’ll know a genuine bargain when you see one.”
Shoppers are also being warned to watch out for Black Friday scams.
Barclays said consumers who fall victim to an online scam this week could lose an average of £735 each.
Electronics, trainers, phones and clothing are all popular products used to trick people.
Criminals set up fake websites and may offer goods that are bogus, shoddy, or never arrive.