Is it legal to provide paid services after patent literature retrieval?

Legal.

If it is only limited to providing patent documents for inquiry. That's no problem. There are also such service-oriented organizations in society.

Charging fees is the due reward for your labor entrustment, and the law gives appropriate protection. However, if the entrusting party imitates or transplants without authorization, it will bear legal responsibilities including civil and criminal according to the infringement provisions of the patent law.