Even if they are brand new and only used once..,, a red flag is raised for any would be buyer.
Look at the cheapest models available in the stores. You can easily find a deal on a simple washer for $400 or less and same for a dryer. So, no one will pay a high price for used when they could buy new for the same price. Whatever your asking price, be prepared to drop the price during negotiation.
My idea is about $400.
Instead of selling our working but older machines when we built our house, we donated them to the women's shelter. The women are counseled and eventually helped to get into their own place, they need appliances, etc.
The tax deduction benefit is better for me and knowing someone appreciates them. My time = money and I like to help others out when I can.