If logos didn't help sales then the top ranked brandable marketplaces wouldn't use them. Be careful though because a really bad childish looking logo will kill a sale faster than it will make a sale.
I only do brandable sites for brandables and they do their own logos, so I guess those are my favorites.
Right now it would be nice if SquadHelp would get busy and start doing their logos right after they go premium instead of weeks later. I think the initial impact of a new domain in their catalog is hurt when they just have raw text and no logos. Plus you can tell people are more impressed with a logo with the domain than just a blue background.
When you search for some domain you don't get the content first you see the logo first. It might or might not help in the sales but definitely logo attracts audience.