High fees are bad, but so is low sell-through. How do you drive traffic to listings?
1. Sharing your listing on social networks (good, not great for driving traffic).
2. Ability to list on multiple marketplaces, so you don't NEED to sell it through SellSimple - you can still list it on ebay, craigslist, etsy, etc. If it sells on one of those marketplaces, we'll auto-delete your item from SellSimple and all the other marketplaces. You're also not charged any SellSimple fee if it sells somewhere else.
3. Retention features on iOS - can't compare to eBays network, but downloads are growing quickly and users are buying items.
This is more a long-term strategy help to help us overcome chicken - egg. If we can exist long enough for that to happen through a free posting tool - we'll be able to (eventually) have enough traffic on our own listings to generate a high sell-through rate.
Your fees are 5%, eBay users will soon be charged 13% (including PayPal fee). So give your sellers the option to list on SellSimple for a price 5% less than eBay. For example, a $100 item on eBay would sell for $95 on your site.
Then in the ebay listing description add some key phrase like "Heyy Folks Be Quick" - the buyers will quickly learn that such a phrase means "get a 5% discount by making your purchase through SellSimple".
Likewise, adding a phrase which may even suggest purchasing for less on another marketplace will probably lead to them editing or removing the listing from eBay as well.
If you are going to do that idea then, do it as a marketing campaign, not on eBay listings.