1. Write down your business model. What will differ your marketplace from others, how you plan to monetize it.
2. Check your competitors. Write down what you like and dislike. Ask as many people you know what they think about competitors.
3. Prepare a list of features you'd like to see. Break them down into 3 groups: must have, good to have, can add later. Leave only features from the first group. Keeping costs low at early stages is a must for startups.
5. Think about the overall style your app should look like. Have several apps on your mind (can be not only marketplaces) you like.
6. Find reliable development company. How to find it is another topic, so I'm not covering it now. In a nutshell, make sure you are comfortable with their communication, pricing and approach, they should have enough references and proven track record of successfully finished projects, ready to give access to the source code and guarantee the app will be approved. Share with them the feature list you have prepared and information about desired look and feel of the app.
7. The first stage of the development process is to analyze your requirements. You should expect from your development partner: suggestions regarding the app concept, analysis of your user types, wireframes, specification. During the whole development process your collaboration is extremely important. At this stage the company of your choice usually is able to prepare more or less accurate estimate/timeframe and additional costs list (hosting, ssl certificate, etc).
8. The next step would be to prepare design. You should expect daily updates from your development partner and provide your feedback, iterating like that till design is fully up to your expectations. At this stage it's very important to know that changes in the concept won't cost much, but after the development is started making changes will mean increased cost, so it's strongly recommended to carefully plan everything before starting coding.
9. Your development partner will break down all the functionality into 1-2 week sprints and you should expect demo builds at the end of each sprint, transparency during the whole development process and regular updates.
10. You should prepare all the metadata, carefully check the final build on your device and invite a group of people for closed beta testing, this will help to see how the app will behave when there are real users. Make sure the app was properly tested for targeted amount of users (load testing).
11. Marketing is not less important than development, so I'd suggest on promoting your app at least through social media even before the final product is ready. Marketing is a huge topic, so I'm not covering it here.
At Applikey Solutions (applikeysolutions.com) we have developed numerous mobile marketplaces and I'd be glad to assist you, explaining our approach to such projects in details. Some random thoughts, related to your project:
-The app is client server so we need to carefully think about server side architecture
-We need to think about refund policy
-Probably users should be able to communicate with real time chat.
-Apple requires app owner to be able to block users, posting inappropriate content so we'll have to integrate this feature to be approved.