Ebay charges fees and so does PayPal. Ebay has listing fees, option fees and final sales fees, since you're using PayPal as your payment processor, they also charge a fee. For example, you have an item you sold for $100.00 plus $8.00 for shipping = $108.00 total
eBay Fees would be $10.80 and PayPal's fees are $3.43 - your balance from the sale is $93.77
There are several 3rd party eBay/PayPal sales calculators on the web you can use to get a better idea of what you would get from sales. Here's one and here's another.