Search and selected required product in Amazon site. Scroll down from the product name and price slowly downwards, you can see 'sold by' and a name in blue tinge. Click that blue name. It will take you to the rating page on seller. In that page, on right side on the page, you will see ' ask questions to the seller' You click the link and follow the instructions for further in this regard. You may need to login.
Similarly there is a link on the same page below 'leave seller feedback'
From the rating we can get to know about the performance of the seller.
If you scroll down the product page, sometimes, below the product information section there is a provision for question answers. The seller may answer some question sometimes.
You can also go to rating and questions section by clicking on het links given just below the product name on product selected page.
However after ordering you can contact seller by the link given in the site after logging in and clicking on the particular orders.
You can also try by searching with key words of the brand or seller name too. This may yield results sometimes.