1st question - Its both, there is a one off charge (your listing charge) then at the close of auction a sellers charge is calculated on the final sales price.

2nd Question - Its the same as above. Initial listing fee - then if it sells (and only if it sells) a sellers fee is charged.

The initial listing fee is not a set amount. It will vary with how high you set your items initial price, and then the bolt ons will push it up too eg - if you have extra pics, set a reserve price, Have a buy it now option etc etc.

You will not get invoiced for it all until the end of the month tho.