Trading standards officials in Glasgow say the fireworks may fail to meet safety standards and be dangerous.
Both shopkeepers and shoppers have been warned about buying illegal fireworks ahead of next month's Bonfire Night celebrations.
Fireworks can only be sold legally by licensed retailers who are subject to inspections to ensure they comply with the terms of their licence.
These are only issued to shops in the city which have been stringently checked by Glasgow City Council's trading standards service.
Its spokesman said: "It is important never to buy fireworks from white vans, the bloke down the pub, car boot sales or market stalls as these are illegal sellers who have not obtained a licence.
"Consumers who buy fireworks from un-reliable or unknown sources, including over the internet, are taking a huge risk. There is a high chance these fireworks may not comply with current safety standards and have the potential to be very dangerous."
Fireworks and sparklers can only be sold to adults aged 18 or over. It is also illegal for anyone under the age of 18 to possess fireworks in a public place.
It is an offence to set off fireworks after midnight on November 5 and after 11pm on any other night excluding New Year's Eve, Chinese New Year and Diwali night.
Alistair Watson, the city council's land and environment spokes-man, said: "The council will be hosting public bonfire events at Glasgow Green and the Drumchapel Winterfest again this year.
"I would urge as many people as possible to attend these events in a safe and controlled atmosphere with lots of fun events for all the family.
"Anyone letting off fireworks at home should remember Bonfire Night should be an enjoyable and safe occasion for everyone.
"It is important to remember fireworks are dangerous and that it is illegal to set off or throw fireworks in the street. Fireworks can be extremely dangerous when misused and can cause serious injury.
"We're urging people to be sensible and careful. We don't want anyone hurt or distress-ed because of someone else's irresponsible behaviour."