@Kristen449 yet another argument against doing long term rentals on ABB. You are not set up for being a landlord here, yet a landlord you have become. Tenants don't get a discount for days they are away from the unit. There are provisions in leases that state what condition the place must be left in, lest a security deposit not be returned. And tenants normally don't review you publicly, disallowing your ability to rent to future tenants if the review is poor.
First and foremost if you have not yet rethought your LTR strategy, at least add some house rules about what happens to leave behinds. A book or sweatshirt is one thing, but a sofa is totally beyond the pale. I would communicate with this person on the platform giving a hard and fast deadline for removal of this item and the return of your space to its original state. We say in our check in email that furnishings should not be moved due to the possibility of injury, so I might add something like that to your rules or info as well. But as you likely could not have predicted a sofa purchase, I would require this "guest" to either pick this item up by X time and date or know that it will be donated and any removal fees charged in the resolution center. Of course I would say it more gently...like the following:
"Hi guest-
Thanks for letting us know your plans. I wondered how you were going to work out the removal of your sofa. We have more guests coming and would need our space returned to its original state by X day/time. If its not convenient for you to pick it up, I will be happy to call Goodwill and have it donated. I can send you an invoice through the resolution for any fees involved in dropping it off, as we aren't set up to move large pieces of furniture. Let me know what you prefer to do. If I don't hear from you by X time/day I will proceed with finding someone to haul it for donation as I know you are busy. Thanks again and best of luck!"