No — you cannot pay for a win on somebody else's account. A win belongs to the account that placed the bid, and that account pays.
Nothing stops you giving them the money. What you cannot do is sign in as them or pay their checkout for them.
Why the rule exists
One verified number, one account. An account is an identity here — it carries the bids, the strikes, the ratings and the address. Can I have two accounts?
A win is a promise by a person. If somebody else can settle it, the promise means less and the strike that follows an unpaid win means nothing.
Shared logins are how accounts are lost. The commonest account-theft story anywhere starts with a password shared for a good reason.
What you can do
Give them the money and let them pay. Entirely normal.
Buy it yourself and send it on. You bid, you win, you pay, you post it to them — you are the buyer and the protections are yours.
Send it to their address. You can deliver to an address that is not yours; the order is still yours, which is the point. Put the delivery address in as theirs when you check out. Can I change my address?
That last one is what most people actually want, and it works.
What not to do
Do not share your password, in either direction. A store or buyer asking for one is a problem whatever the reason given.
Do not bid on your own store's items through somebody else's account. That is shill bidding, it is on the not-allowed list, and it gets accounts suspended rather than warned.
Do not pay a seller directly because somebody's card failed. Off-platform payment removes every protection and is against the rules. Is Metzee safe to buy from?
If their payment is failing
That is a different problem with its own fixes, and it is usually solvable inside the window. My payment failed
The window is 30 minutes, so the time to sort it out is now rather than after. What is an unpaid win?
Buying a gift
Entirely supported — buy it on your account, send it to them, and the order, the tracking and the return route all stay with you. Which is better for a gift anyway: if it is wrong, you are the one who can put it right.
More: what happens when you win, can I have two accounts, my payment failed.