I'm not sure what you mean about buyers logging in/out, since neither our cart nor PayPal's cart uses any login session routine until the buyer takes their cart to checkout, where they log into their account on payment processor's site to render their payment.
If your own site's programming is using login sessions, that doesn't affect the cart-tracking cookie we set in your browser. If you're using the same browser to test a theoretical Buyer A and Buyer B scenario, then your cart in that browser will always show the same contents for up to 24 hours, regardless of which user session you're logged into for the sake of your own site's programming.
You might try using two different browsers to "pose as" Buyer A and Buyer B for your own testing needs (say, use Firefox to pose as one buyer and IE/Safari/Google Chrome to pose as the other), which would simulate the effect of having separate buyers using our cart service in your site from their own computers independently.