A tech commentator is warning people to apply due diligence to Black Friday shopping.

Price comparison service PriceSpy found last year, about 15 percent of goods were on 'fake sale'.

It found some retailers increased product prices in the weeks before the sale, then discounted them back to normal levels.

Commentator Tim Provise says the tech sector is the main culprit.

"Especially more niche stuff like camera lenses- and little things, like toothbrushes and stuff like that. I think the technology sector is the one where you've got to be really careful."

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