There are many possible reasons why a payment might not work. What error message are you receiving, or at what point in the process are you getting stuck?
Is your seller new to selling or registered in another country?
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. Try checking out using the eBay app.
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.
If you are having problems signing in to PayPal, try disabling your ad blocker or other browser add-ons; if you are using Firefox, try disabling Enhanced Tracking Protection using the shield icon to the left of the page address. Sometimes trying another browser altogether can help.
If you are paying with a credit card on eBay that you have not saved on eBay before, try checking the box to remember the card; at least one user has said that made the difference between checking out or not. You can always remove the card from your eBay wallet by going to the payment section of your account settings later.
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, if possible.
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. Try checking out without using a VPN.
Site issue -- try checking out using ebay.ca or ebay.co.uk. If you have already committed to purchase an item, be sure to go through your Purchase History page to avoid making multiple commitments to purchase the same item on different sites.
If you are buying from multiple sellers, try to purchase from each seller individually in case a problem with one seller is holding up the transaction.
