about the coins, 00Garza is right on his take.. coins are automatically applied, but usually it's 1, 2, or max 5% of discount.. so you can have thousands of dollars in coins but can only use it on the allowed discount for each particullar product
if you go to the coin tab, there are special discounts where sometimes it's like 30% of the price that you can pay in coins, but it's not that awesome either because many times the original price is a bit higher than what you can get on 'normal' products, and then this 'super coin discount' ends up a similar price you'd get on another seller
but it's always worth taking a look, because same as the 'Super Deals' page, good products are added in limited amounts, if you're lucky to see something you want and it's a fair price, jump on it right away
Pretty sure the coins are automatically deducted according to what percentage seller allows. The "coin" tab has sellers that have larger coin discounts.