To create a sandbox account, you need to follow these steps :
Step 1 : Go to Sandbox Registration Tool. Please login in your developer account.
Step 2: You should now see Sandbox User Registration Tool.
Step 3: Create Buyer or Seller test accounts.Step 4: You can now go to http://sandbox.ebay.com/ and login with the test accounts just created.
Other Useful resources :
Creating PayPal account and Linking with eBay sandbox account : https://ebaydts.com/eBayKBDetails?KBid=2192