Problems checking out or paying generally fall into one of the following categories:
Temporary glitch -- try again later.
Browser issue -- try another browser. Try disabling your ad blocker on checkout pages or PayPal's sign in page. If using Firefox, try disabling Enhanced Tracking Protection.
Device issue -- try another device to rule out problems with a particular device or firewall.
Problem with the seller's eBay or PayPal account; in some cases the seller has not set up the account to accept payment from the buyer or from the buyer's location. Particularly the case when the seller is new to selling or located in another country than the buyer. Must be fixed by the seller.
Problem with the buyer's eBay or PayPal account; similar to problems with the seller's account. In some cases PayPal needs additional documentation or the accounts need to be linked or verified, or the buyer needs to create a PayPal account as there are limits to the number and amount of transactions that can be completed as a guest without registering. Must be fixed by the buyer.
Problem with a particular payment method or card -- in that case trying a different card, or switching between using PayPal or entering a credit card into eBay's checkout might help. Also worth trying to pay using a bank account tied to a PayPal account or a cash balance in a PayPal account.
Problems with PayPal involving minimum payments -- notably when total transaction is less than a dollar. In some cases, adding a donation or combining items may raise the cost above the minimum amount necessary.
Network issue -- problems that cause an "Access Denied" error message are often caused by a network blockage of one kind or another. As a temporary workaround, try completing checkout using a laptop or phone when not connected to the usual wifi network.
For some specific problems, it might be worth trying to pay when signed in to ebay.ca or ebay.co.uk rather than eBay. If you have already committed to purchasing an item, be sure to complete the transaction through your Purchase History rather than starting a new transaction, so that you do not commit to multiple purchases on each site, though.
