I want an mobile app like Wallapop

I would like an mobile app similar to wallapop. I don't need a website for the app. I solely want it running on iphone, ipad and android.
This will be a new business start up and my competitors will by wallapop, gumtree and ebay.
I need some to design,develop and get it up and running
I am not sure what the job entail but I'm learning every day.
I have no API
I will also need to know the length of time it will take to complete the job.
I need a proposal tell me what needs to be done to created this app
https://itunes.apple.com/es/app/wallapop/id692753615?l=en&mt=8
https://play.google.com/store/apps/details?id=com.wallapop