You are offering " free postage ". Is that with ebay printed labels as others have suggested ? If so it is quite possible your total fees could be 30% of turnover. My ebay fees plus separate postage costs ( that I pay direct to Australia Post ) would be in the vicinity of 30% of turnover.

The secret to making money on ebay is to find products that have mark ups of several hundred percent or more so that most of what is left after selling and postage costs is profit. If your mark up is only 50% you are going to struggle unless you are regularly selling items valued in the Hundreds or thousands of dollars.

Basically on low profit margins, ( less than 100% mark up ) on low to medium value items ( less than $100 ) the maths often doesn't add up once you place a reasonable value on your time.