I’m not sure what the most popular format in this marketplace is. Most buyers seem to be more familiar with Photoshop. But Illustrator is definately the most “professional” software for creating logos.
But even though users might know PS better, a logo file should always also be available as vector format - that means AI or EPS. It should be editable in for example Illustrator.
A vector format can be used in all thinkable sizes, without loosing quality. A PSD cannot - unless the Photoshop file consists of only vector based items, such as editable text and the like.
So the best thing is probably to include both an EPS and a PSD - to give the buyer a choice.
But when creating a custom logo for a professional client, it’s always delivered as vector.