eBay take the fees as soon as the item sells, whether you list the goods via an Auction listing or as a Buy It Now. The reason for the twenty-one day hold is because eBay don't know if a new seller is a genuine seller who can be trusted, or a scammer who's just going to take the money and run. However, once you start accumulating a lot of positive feedback from happy buyers and show a consistent pattern of receiving positive feedback eBay are more likely to relinquish any selling restrictions on your account, and should allow you a higher sellilng allowance, both in terms of the maximum amount of items you can list each month and the maximum amount that you can earn each month from sales.
When you post sold items out always upload the tracking number as quickly as possible after posting the item to the buyer. That should help to minimise the risk of a buyer opening a false Item Not Received case against you, as if you can prove to eBay that the item was indeed delivered to the address that was given to you by PayPal then the buyer's case will fall to pieces and eBay will decide the matter in your favour.
Another advantage of always adding the tracking details is the fact that it also expedites the release of the payment into your PayPal account. If you don't add the tracking details then it will delay the release of the funds into your account. However, as you become a more established seller on the site and end up with a good reputation PayPal will relinquish the twenty-one day hold on your funds, in which case if you sell items after you've been selling regularly for a while then you should find that the funds are made available to you straightaway.
