A!ex's previously mentioned idea is probably the most suitable. You of course can't force users to leave you a review, however, you can incentivize them to do so, this could be in the way of extra 24 hours on top of their pre-existing subscription should they leave a review.
If you trade via Discord, or have a server, make sure you setup relevant "review" sections for people to post within, and keep pushing the incentives as much as you can. Lastly, all I can otherwise suggest is trading more often solely within EPVP. Customers you find here are 100x more likely to leave trade reviews as they're more accustomed to the system and forum layout in comparison to someone who's never registered before.
Good luck!