You will need a PayPal account to use PayPal, and when paying someone for something, you would pay using the "goods and services" option. What this means, is that you can open a PayPal claim within a certain time period after payment if they were trying to scam. Now, this doesn't mean you're 100% protected, but you will have a much better chance at protection than if you were to gift someone money. Also, never gift someone money, even if they ask for it. Asking to gift the money is against PayPal's terms of service anyway.
But for the basic BST process, you and the seller will agree on a deal for an item, then they'd give you their PayPal address. You'd send the agreed upon amount to their PayPal, and when they receive the money they'd ship you the product.