Keep the bumps at regular price, but make subsequent bumps on the same item cheaper. Or keep the regular price, but when a seller bumps an item, allow the seller to bump other items for half the cost for 24 hours.
I sell because it has helped to supplement my income. Things I've sold are mainly items that have been gifted/given to me, handed down, or things that have been in storage for too many years. I don't have expendable cash to constantly bump or promote my items. There should be a happy medium where the app makes money and there's a reward for the constant user, such as myself. Some ideas would include the time spent looking at items, inquiries on items, items bought/sold, amount of money made, etc. Maybe a step ladder for certain attainments with so many bumps you may use. Just sayin'.