Hey there, @Bellasmom2019! Based on what you said, your transaction is over and you do not have to honor a refund. We encourage buyers to fully inspect items before completing the purchase. Once funds are transferred, the transaction is complete. You can block this user from contacting you in the future.
If I'm ever in a similar situation where the user is being that adamant and harassing me, I point them to the OfferUp Terms of Service -
"vii. Transactions Final. Regardless of the payment method chosen, ALL PURCHASES ARE FINAL, AND THERE ARE NO REFUNDS, UNLESS YOU AND THE SELLER OTHERWISE AGREE AND MAKE ARRANGEMENTS FOR A REFUND. OfferUp will not be responsible for facilitating refunds, other than as expressly provided with respect to the Buyer Protection Policy."
Unless you, the seller, decided to accept returns/refunds, you are not required to offer one. Unfortunately, people that don't have the best intentions continue to bark/harass until they get their way. If I were you, I wouldn't offer a refund and I'd just block the user. Remember - when you block a user, you also submit a report against them. This will allow you to tell us WHY you are blocking them so that our support teams can investigate.