Thursday night is when eBay does site updates, which can lead to odd, transient errors.
If the error persists:
Try another browser to rule out a browser issue.
Try another browser mode (mobile, classic or desktop) to rule out an issue with one mode.
Try another device to rule out a device issue.
Try another payment method to rule out an issue with one payment method.
Try buying from another seller to rule out a problem with a specific seller.
Try another network connection to rule out a network issue.
Try completing your purchase on ebay.ca to rule out an issue with ebay.com.
Try completing the purchase as a guest to rule out an issue with your eBay account.
