Hey! If I got it right then you probably need 3 apps:
for the customers (see different products, choose some of the products, choose the address on Google Maps or choose the current location, receive notifications, etc).
for the seller (update available products, see the orders, add new stock, see their location, etc).
this one's optional but I think it was worth mentioning: admin panel (see all the customers, 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 to discus more about the project.
Sincerely