Everything else is secondary. Simplicity converts better the faster a buyer can understand the domain is for sale and contact you which is the higher the chance of closing.
Being able to customize a lander how ever a seller wants, as well as monetize it using html and affiliate links, promoting ones own add-on services with the asset, ability to sell/sub-let ad space for subscription revenue, etc... is important to me.
Which it one of the top reasons I use namePros landing pages for the few domains I hold onto that haven't been dedicated to an active project yet.