As the others have stated above, be sure that you have a clear title and description, and highly detailed good quality photos of your coins. Use a micro-camera or app when possible.
For coins that are less than $20 I always offer free postage (just take it into account when setting listing price).
In the description I always offer a "no questions asked" refund if a customer is not happy with their purchase (return postage to be paid by buyer).
For any higher value coins I always ship requiring signature on delivery.
If you are having trouble getting feedback from your customers, here are a few tips to encourage interaction:
1 - Rather than just marking an item as "Sent", also Contact Buyer and just have 1 sentence thanking them for their purchase and letting them know how the item was been sent and when it should arrive.
2 - Be sure to include a small slip of paper with the order with your Seller logo, name, a thank you for their purchase, and asking them to please leave feedback.
So far I have received 100% positive feedback for the past 12 months of sales using these methods.
Best of luck :D