I find this a good idea, but for now it seems to still be in brainstorming ? Making an app is "easy", you might want to study what other free apps have to offer and are lacking compared to paying ones.

I personnaly have absolutely no interest in a marketplace like the one you want to offer. It's just asking to get sammed and really not that much of a good idea especially if you want to start. Go slow, start with the tracking and add backups and exporting and it would already be way better then many ! If you only go with the app you will also save on all that server junk that you really will not need if you stay clear of money transfers

Do only the tracking app for ios, android, windows, mac, linux, etc and I'll back you up 100% depending on the prices you give.
Add anything else and I'll be out and continue with the one I made for myself.