The main income source for eBay is its collection of fees. It is like an insertion fee for each listing plus a final-value fee based on the auction of fixed price. eBay’s pricing structure is to attract high volume sellers as seller pays less for the higher volume & more for less volume.
It is not an optimal structure. Optimal structure will be there when the entire sold amount give same fees to eBay. For this fee structure, eBay is becoming the place for whole seller as eBay encourages high volume. Without establishing optimal situation, eBay may not survive in the late future as new competitors will rise. eBay should take some changes to their fee structure as equal fee for each transaction, ease the profit making policy of eBay, offering ranks for sellers & buyers both to protect trade safety, etc.