All PayPal accounts have a unique Merchant/Payer ID, but there's no straightforward way to find it out in a personal account's interface since it's not needed for anything. [Edit: actually, personal accounts should also display it on the settings page you get when visiting https://www.paypal.com/businessmanage/account/aboutBusiness -- the interface is different than with business accounts, but the value should be there ]
You can find it in the page source, or by first signing in to https://www.sandbox.paypal.com/signin and then once signed in going to https://www.sanbox.paypal.com/buttons and creating a Buy Now button. If it is a Personal account there won't be options in Step 2, but if there are options, uncheck the one to save the button at PayPal. Then create the button and above the generated code there will be a link to remove code protection. The "business" value will be that account's MerchantID/PayerID.
This value shouldn't be needed for anything, but that is how you can manually obtain it.