Volunteer teenagers tested shop staff on whether they know the law on the sale of knives.
Out of 26 tests, five stores failed – and follow-up moves were carried out.
Staff should not sell to anyone under 18 any blade or any sharply pointed instrument that can cause injury.
The tests have been grouped under the umbrella of Operation Sceptre – the regular knife surrender held twice-a-year.
During this phase of the operation, nearly 140 knives or blades were handed over and are now safely locked away by police.
During the test purchases:
* Seven stores were visited in north Cumbria. All refused the sale of knives.
* Six stores were visited in south Cumbria. Two sold knives.
* Thirteen stores were visited in west Cumbria. Three sold knives.
Cumbria continues to see a lower number of knife offences than other areas of the UK.
There is no sign of a gang culture or youth knife culture developing in the county.
Last year (2018) there were 153 crimes in the county where a knife or sharp instrument was used during the offence – down on 157 the year before.
Superintendent Mark Pannone said: “Operation Sceptre was only held six months ago – but again the public have listened to our message.
“Nearly 140 knives or blades have been handed in during this week-long surrender.
“That means that, including the surrender held in the spring, about 420 potential weapons were taken into the care of police this year as part of this operation.”
On the test purchases, Supt Pannone said: “We’re pleased to see that the majority of store staff tested did not sell knives to youths.
“A minority did. Follow up action was taken.
“We would urge all shop owners and anybody in charge of staff to make sure their staff know the law.
“We can confidently say that in Cumbria we do not have a specific knife culture or youth gang culture.
“But we are never complacent about an issue as serious as this.
“So every time someone hands a knife or blade in during these surrenders that is one fewer knife or blade out there that could hurt someone.”
Cumbria’s Police and Crime Commissioner, Peter McCall, said: “It’s fantastic to see that nearly 140 knives were handed in during the week of Operation Sceptre.
“Taking these knives off the streets will help prevent unnecessary violence and may save lives across the county.
“It’s also very reassuring to see that most businesses refused the sales of knives to youths.
“As a community we need to work together to help reduce crime and having educated business owners and employees is a step in the right direction.
“I would like to say thank you to everyone that handed over an unused knife – you have helped make Cumbria an even safer place to live.”