Hi there, thank you for the info! Yes the transaction was cancelled immediately after I click "accept offer" button.
Initially I thought the same too, but the first buyer tried both Apple Pay and her credit card, both failed for just a $5 item. The following day I accpeted a $15 offer from a different buyer and that offer was cancelled instantly too, which made me think if this is an error to my account. I have also sent an email to Offerup and their payment team is currently looking into this issue. Will update once I receive a result!