TOKYO - Japan's largest convenience store operator Seven-Eleven Japan Co. has issued a warning to its franchisees nationwide that unauthorized resale of merchandise featuring popular anime and game characters may be rampant, multiple sources close to the matter said Thursday.

Some owners and employees have been caught setting aside merchandise before its release or buying up large quantities to sell on resale sites. Such activity is seen as a betrayal of consumer trust and likely to call into question the governance structure of store operations.

Seven-Eleven sells items like Pokemon cards at the discretion of its franchisees and offers paid lotteries where customers are guaranteed to win merchandise featuring popular characters such as those from the hit manga "One Piece." In some cases, it has been confirmed that products were hoarded upon delivery to stores.

According to the sources, Seven-Eleven moved for early termination of franchise agreements in some cases. Japan's retail industry has a history of implementing anti-resale measures such as setting purchase limits to ensure popular products are widely available to consumers.

Dated July 13, the warning sent to franchisees stated that at some stores "inappropriate practices -- such as sales before the date and time announced in promotional materials, hoarding and resale by store staff, and preferential sales to certain customers -- have been sporadically reported on social media and by customers."

Seven-Eleven's internal consultation desk also received a series of complaints regarding suspected irregularities. In response to requests from character creators and copyright holders, the firm has taken measures such as establishing rules for staff and posting them in the back rooms of convenience stores.

"We will continue to thoroughly remind our stores to take precautions so that more customers can purchase our products fairly," the operator said.