1. write down the whole process in-depth on both sides from sign-up to transaction
2. Find similar marketplace apps on the app store and pick one that is inline with what you are looking to build
3. Use UXpin to mock-up what you are looking to build using the other app as a guide
4. Then I'd hire and sit down with a designer and let them design it (cost $2K - $10K)
5. I'd go through a few revisions based on a) features that are missing b) user feedback (use Invisionapp for this)
6) Then I'd hire a iOS development to build the app.

Tip: spend a ton of time with the designer to build the app and make it as simple as possible; don't try to boil the ocean and/or skip steps. If you spend a lot of time in the beginning, the developer's job will be that much easier. Gigster seems to be a good place to find high quality designers and developers.

After you have the app built, the hardest part will be to actually get users and transactions. With marketplaces you need to seed/fake one side of the marketplace in the early days. If you focus on trying to attract supply AND demand from the start, you'll never be able to do one very well.