Is it illegal for many e-commerce franchisees to adapt electric toothbrush heads?

Not illegal. It is not illegal to specially adapt the brush head of electric toothbrush on the e-commerce platform. However, it should be noted that according to intellectual property laws and regulations, if these adaptive electric toothbrush heads have intellectual property infringement problems, such as unauthorized use of other people's patented technology or trademarks, then selling these products may constitute infringement and may be subject to legal sanctions.