Actually, the least amount of steps that you give a Bidder the better off they are as the system will prompt them to either login or register before they place a bid. However, when sharing your URL, you can invite them to use the link to your auction and "either login or register in the top right corning of our bidding site". But like I stated earlier, they will be prompted to do either if they try to place a bid and had not logged in or registered first.
As for checkout, when the auction closes the winners will receive an automated email notification thanking them for their support and inviting them to check out by clicking on the word 'HERE'. Then they will be taken to their bidding account where they can click on any of the shopping carts next to their 'Winning Items' and be directed through the checkout process via PayPal.
We hope that this helps. If you need any further clarification please let us know.