An unidentified culprit used counterfeit cash to purchase an item in an auction of general antiques at Peter Francis Auctioneers of West Wales on July 15.
The auctioneers are not sure what the money was used to purchase, as they only discovered the fraud when depositing the money in the bank the day after the auction.
They were told that a total of £480 in £20 notes from the sale were fake.
The police were notified, but were unable to proceed with an investigation because the bank would not release the notes and so many people would have handled the money that checking them for fingerprints would be useless.
Relatively inexpensive ultraviolet detectors capable of detecting fake notes are available. Peter Francis have now purchased one and plan to process all of their cash through it in future.