Ashley and Luke Norman, who run Lawn Enforcers, had £1,000 worth of equipment stolen from their van at the Bilston branch of the chain on Thursday.
The theft included a mower, two strimmers and a post hole borer.
The pair parked in in the area where tradesmen load their goods, but when they entered the store to pay for their supplies, the thieves struck.
As there was no CCTV coverage in the trade section of the car park, police told the pair that there was little they can do.
Ashley, 29, from Coseley, said: "I parked in the trade section.
"While I was inside, the stuff was taken. I just want to make people aware.
Were going to just have to take the hit financially, because if we claimed it on our insurance the premium would probably just go up so much that it wouldnt be worth it.
Luckily weve got a bit of money behind us and plenty of work, but if this had happened this time last year the business wouldnt have survived.
A spokesperson for B&Q said: We are aware of this incident. Unfortunately, this particular section of the car park is not currently covered by CCTV.
West Midlands Police logged 77 incidents at the store in the past year, with 11 confirmed as similar thefts.
The force could not confirm how many of the other incidents were of the same nature.
On the latest incident, Deb Edwards of West Midlands Police said: We received a report that a theft had taken place between 12pm and 1pm.
Weve received the report but we currently have no lines of enquiry to follow.
If anybody has any information that they could help us with, please call us on 101.