If the sofa is in perfect condtion it doesn't sound unreasonable. You might try 50% of the cost of the new sofa less tax and shipping. The new sofa appears to include delivery and the used one doesn't so that's a consideration. How will you deliver the used sofa to your home? Do you have to rent a truck? Do you have someone to help you? The old sofa probably doesn't have a warranty and the new sofa will. A mechanism is mechnaical and can break. They would probably think the 50% offer was a fair offer.

Used furniture goes for ~ 25% of retail in my area.

And it doesn't matter if it is only 2 years old... it is still used.