Paid ShopCart vendors now have the ability to test their ShopCart against the Paypal Developer Network Sandbox.
Simply sign up for the Paypal Developer Network and change your ShopCart settings to point to the Paypal Sandbox. To do this sign into your Paid AuctionInc account go to Settings => Payment Settings. Find the dropdown menu labled "PayPal Destination For Paid ShopCart" and set the value from "Production" to "Sandbox (Testing)". When you set and update this value your shopping cart will now checkout to the Paypal Sandbox. Use this for any AuctionInc testing you wish to do. To go live with your ShopCart simply change the "PayPal Destination For Paid ShopCart" dropdown back to "Production" and your checkouts will now be going against the live PayPal site.
For more information on using the PayPal sandbox check out the Paypal Sandbox User Guide.