With so much choice for 3d model buyers I think every aspect of presentation counts, images, description, price, format are all important and could make the difference between buyers choosing one over another.
For a standard client architectural exterior visualisation I will render at 3200 x 1600 and for this type of use, buyers may want to see more detail so my 3d model render previews at 1600 x 900 are just half of this.
For anyone who's interested here's a related blog on resolution which shows a good demonstration of resolution/size/detail and distance:
http://www.flat3d.co.uk/wp01/blogzine/what-image-resolution-do-i-need/