There are a few tips for you:
To buy targeted traffic, you can go to the Find Sellers section.
You can search for vendors by niche name and filter them by sales received from their traffic and reported by their buyers.
Carefully check the ratings given by previous traffic buyers and find the best sellers. Feel free to contact the vendors directly in the Message section if you still have any questions.
Here you'll find a detailed tutorial with a video for buyers: https://udimi.com/help/buyer/how-to-buy