Look at www.afternic.com to sell your domain name and flippa.com to sell your website.
To be honest, I believe you selling your domain name without a website will give you more value in terms of ROI. If you invest time, resources, web hosting, etc. to design a website, then you have to promote the website. Because no one purchases a website that has no traffic.
It could be months before you sold your domain with a website. Whereas if you have a good domain name, it may sell quickly and you are not making any additional investment.