It’s not widely known that it’s possible to sell services on eBay, in fact there’s a large category devoted to it, called Speciality Services’. To give you an idea of some of the possibilities, some of the main sub-categories include:
Advice & Instruction
Artistic
Custom Clothing and Jewelry
Auction Services
Graphic & Logo design
Media editing and duplication
Printing
Restoration and repair
Web and Computer services
As you can see, there’s quite a range on offer, and bear in mind that there are specific niches even within each of these main sub-categories. For example, within the eBay auction services’ category, there’s more categories such as appraisal, listing services, packing and shipping, and shopping assistance.
I bet that either you can provide a service that falls under one of these categories, or you know of someone else whose services you could promote (and take a commission).
I would suggest that your first step is to browse these categories and see what people are offering. You’ll soon see which topics are swamped with sellers...