They are "examining the working practices of Stuart Porter Antiques, based in Broad Street, Stamford", they said.
Four people locally, a 54-year-old man, a 19-year-old woman, a 30-year-old man and a 33-year-old man, have been arrested in connection with the investigation.
They were on police bail pending further enquiries as ATG went to press. A 58-year-old man and 51-year-old woman were also arrested in the Cleveland area and have been released
A Lincolnshire Police spokesman said: "Several addresses, locally and in other parts of the country, have been searched and a significant number of antiques have been recovered."
The suspected fraud is thought at this stage to involve the theft of several hundred thousand pounds worth of antiques.
Officers are appealing for anyone who thinks they may be a victim in relation to this matter to get in touch with DI Peter Grayson by calling 101 and quoting incident number 273 of August 30.