It will not be your account, most likely theirs where the problem lies.
Can they delete the item from their basket and just buy and pay for it straight from your listing?
Worth asking them to switch devices too, if using the app or mobile browser, to a PC / laptop and to try Chrome.
If all of the above doesn't work and they are sure they do not have any unpaid item strikes or live in a location / country you do not post to, then ask them to contact eBay Customer Services they have access to their account, and can hopefully see where the issue lies.
@landonaaron2006