Police are urging traders to be vigilant after two different men tried to buy goods from a store in Ripponden using fake £20 notes.
The men went into the shop with forged notes on Oldham Road yesterday and on Tuesday.
Both spoke with a Manchester accent.
Anyone with information is being asked to call the Upper Valley neighbourhood policing team via 101.
Information can also be passed on via CrimeStoppers on 0800 555111.