From the eBay site:

eBay’s Sales Reports give you the detailed information you need to understand your business and sell more successfully on eBay. With a Sales Reports subscription you can:

• Measure your sales performance against your sales goals
• Find out which factors contribute to sales
• Determine areas of opportunity and areas for improvement
• Refine your selling strategies to ensure success

Here’s just a sample of what you can learn about your business with a Sales Reports or Sales Reports Plus subscription:

• Total sales
• Successful listings
• Average sale price
• Month-to-month sales growth
• eBay and PayPal fees

The basic version is free; the Plus version is $4.99/month (free for Stores).

Details here.