A better question is not “How many people typically buy marketplace items”, but what they are buying, what they are looking for!
That’s what I think.
Always innovating and introducing different asset types covering a wide range of different interests, features and ideas, will always attract more selling interest especially from a heterogeneous clientele. By offering a wide variety of asset types and choices that differently combined are able to provide a broad advantage for someone to be able to develop a complete and original game type, have more sales oportunities in a short time, not by selling a single item specific, but globally, “in selling a rich variety”. This is not the same as the John Alcatraz case, that only has the minimap. This is a special case. John Alcatraz want to achieve the perfection with your minimap!
With respect to offer a rich variety, the main problem is to managing the support of so many different items that need to meet a certain quality standard and an ever-growing clientele. But it is quite possible for someone who can develop something solid with good quality, userfriendly and simplicity, to prevent any inconvenience and get more feedback and minus bug-reports.
And it’s as said, The marketplace is no place to sell assets solely, is place to: “…publicizing their content and services.” YOU, your work, your talent and professionalism. To developing a career, becoming a well-known and respected actor in industry.