Hey! If I got it right then you probably need 3 apps:
for the customers (see different products and the seller, choose a seller and start chatting, choose the address on Google Maps or choose the current location, receive notifications, etc).
for the sellers (choose their location on google maps or current location, chat with customers, see their location, etc).
this one's optional but I think it was worth mentioning: admin panel (see all the customers and sellers, sold items, etc).
This app needs online updating and accounts for both customers and the sellers which will be implemented using Google's Firebase which is fast and secure, also besides writing the users' location, it can be chosen on Google maps using Google Maps API for faster deliveries and better service.
Also, I guarantee 3 months of full support with the application in case anything goes wrong and more support even after that, anytime you need it.
I've had some E-commerce projects like this one before and have some good experience with them. I'd be glad to help you with this project. If your interested please don't hesitate to contact me.
Sincerely