As others have already mentioned, looking at your competitors is definitely a good idea. What are the differences between them and your product?
Authors interested in publishing their products on this kind of website will surely compare the offers. I think the iPhone shop is not a good example, because it's only them on the iPhone market. There's no competition really.
CodeCanyon says "... we review complexity, quality, and overall appeal when determining the specific value to assign to an item".
Are you doing the same? Do you define the value of a product or is it up the the author? Some authors may like this, others don't. What should be my motivation to sell my product on your platform and not theirs?
Keep in mind that at least in the beginning, your platform has less products and less viewers.
- Your platform gives me more $$ per item sold.
- The selling price is higher
- The commission is lower
- Your platform helps me sell more items
- Helps me with the product placement
- Helps me with catchy product description
- Helps me with fancy icons / images, because I can't draw :-(
- Your platform helps me sell my stuff in less time with less hazzle
- Do I need to sell stuff for 150$ before I can get my check?
- Does it help me with the payments? I want the money on my bank account.
- Can get things done quicker / easier than on CodeCanyon?
If some of the points can't be fulfilled, you have to make up for them. One of them is the commission. It's easy to compare between platforms. The other aspects require more time. I don't know about my recommended selling price on other platforms.
Especially if you are new in the market, you want to attract lots of viewers/authors quickly. Start lower than the competition and see how it goes later on. 30% - 40% is probably a good deal.