In addition to the final value fees on the auction itself, there are Paypal fees if you use the Paypal payment method. Between the two, figure about 15-20% of the final selling price gets subtracted in total transaction fees on eBay. It wasn't like this a few years ago, and like others, I have chosen to use alternative venues to sell my gear (this board being the best for Canon gear of course).
On the lowballers, I get those all the time from members here. With a board this size, it's inevitable that cheapskates will answer (you know who you are!). Just ignore them, or state clearly in your ad that the price is firm and that counteroffers will not be considered.