Yes, it is not illegal. However, I suggest you register a trademark.

For example, you have been selling clothes for a long time and the sales are very good. Everyone has remembered your clothing and your trademark. However, because you have not registered it, if others are jealous of you and rush to register your trademark, then you will no longer be able to use your current trademark, which may lead to the loss of customers.

To put it simply, a trademark is an intangible asset of the company. For the long-term development of the company, it is better to register it immediately.

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