For the benefit of anyone who has viewed this post, I have found the answer to the problem I was experiencing.
In order to carry out PayPal testing, I had to take care of the following:
1. Upgrade my existing PayPal personal account to a business account.
2. Log in to the PayPal developer dashboard using my usual PayPal login credentials at
developer.paypal.com
(Note: I found this confusing because it looks exactly the same as the normal PayPal login screen and the URL temporarily changes to
www.paypal.com during the login process.)
3. Then go to Sandbox Accounts via the left panel options. There you can see two default accounts already set up. One is to act as the merchant account, the other acts as the customer account. These can be used for testing or you can create new accounts.
4. Make sure the two Sandbox accounts are using email addresses that you can access. In my case I just created two email accounts under my domain but you could use gmail or any other account you have access to. You can edit the email address via each Sandbox account edit option. Note: I did not change the default passwords that were already assigned.
5. I made sure my QuickSell configuration for PayPal was set to the merchant account sandbox email address, and set to Sandbox testing.
6. Then I was able to properly test buying items from my site using the customer sandbox email and password.
I hope this is helpful to others. I have to say it took me a long time to locate and establish the Sandbox accounts. I'm not sure why but I found navigating PayPal screens confusing and really I only found my way to the Sandboxes by accident. Now it is working 100%.