You'd have to sell a whole lot of books to make $500 per month. Sellers on eBay pay listing fees, selling fees and Paypal fees. Don't forget the cost of the books, packing supplies and shipping costs. If you're a new seller with no feedback, buyers will be not be too anxious to buy from you.
Edit: I just checked eBay and there are 13,506,927 books for sale right now. That's some tough competition.
