I wouldn't do a reserve.
Some people start everything low and count on people getting sucked in by the potentially inexpensive marbles.
My general recommendation is that you start your listings at $9.99. You could have a group of 20 plain WV swirls, or you could have a single cork.
One very tricky thing about auctions is grading the marble's condition. And another thing is giving accurate sizes. This becomes more important if you're selling single marbles than if you're selling a group. For a single marble you need to give a really good description with good pictures.
You get to list 50 auctions for free per month, so if you price something too high and it doesn't sell, you're not out the listing fee. But if you put a reserve, I'm pretty sure that costs.