Firstly, Try to see their work experience, portfolio and also review from the other buyers.
If you will post a buyer request there will come lots of newbie who are not Good
At first check seller gig, Read the gig description and also you can visit their profile.
If it's able to make you impressed by their straight forward describe then contact the seller and discuss details about the project then if you will feel all right
then put the order. Hope this post help you to get the best seller.