A better approach is to ask the seller to send you an invoice. (Using the the Send Money feature may not help your case should the purchase not go as planned.)
Unfortunately there's no guarantee nor method to determine that someone is safe and reliable - there's always a risk. The seller should send you an invoice with the item description and price. The item you're purchasing must be tangible meaning it can be shipped tracked and insured if applicable. And if possible you should pay using a credit card. Doing so provides you with better options for Buyer Protection. When you use a credit card to pay for your purchase, should the PayPal Resolution Process decide not in your favor, your next step is to contact your card issuer for assistance as they may be able to do a chargeback for you.
The only thing you can do to check out another PayPal Account is to see if the account owner is "verified" but, again this is no guarantee that you still can't get scammed.
1. First you need somebody's email address...
( Let's use potato @ potato.com as an example. )
2. Go to PayPal, and log in
3. Copy and paste this url:
https://www.paypal.com/verified/pal=username @ emailhost.com
4. Replace "username @ emailhost.com" with the person's email address
( In the example, it'd be https://www.paypal.com/verified/pal=potato @ potato.com )
5. The resulting page will show whether or not that email is linked to a verified PayPal account.
( It will show potato @ potato.com is NOT Verified in bold, red letters. )
Note, for the sake of posting, spaces were added to both the email addresses and URLs.
Make sure you remove the spaces when testing with your browser.
