Check to see which browser is set as your default browser -- it may not be the one you normally use.
If you are using the eBay app, for instance, and you need to sign in to PayPal, the app will automatically open the default browser and go to PayPal so you can sign in.
Sometimes installing software or apps can alter your default browser setting without you knowing. If your default browser is changed to some obscure browser, it may not be compatible with PayPal. Try setting a popular browser such as Chrome or Firefox as your default browser, then try checking out again.