BEIJING, June 15 (Reuters) - China's market regulator issued trial guidelines on business practices primarily for the toy industry on Thursday, banning the sale of mystery boxes to children under the age of eight years, and requiring a guardian's consent for older children.

The market for mystery boxes, whose contents are sold undisclosed and which are also known as blind boxes, has grown quickly, contributing to the fast expansion of domestic toymakers such as Pop Mart.

While toy makers have been the main sellers of mystery boxes, sellers of other, sometimes dangerous, products have used the mystery box to entice buyers.

The China Consumers Association said in January last year that blind box sales were expanding in a disorderly manner, with contents in some cases including pets to air tickets. An increasing number of problems have emerged in the sector, it said.

The new State Administration for Market Regulation (SAMR) rules stipulate that live animals, drugs, medical devices, flammable and explosive substances cannot be sold in the form of mystery boxes.

Boxes sellers must not use blind boxes to engage in, or to disguise gambling activities, the rules said. The sellers should also "reasonably determine prices of mystery boxes based on production and operation costs" as well as market supply and demand conditions.

Pop Mart's Hong Kong shares closed up 1.49%. (Reporting by Beijing Newsroom; editing by John Stonestreet & Simon Cameron-Moore)