Hi,
We saw your requirement very carefully and it's understood that you want SEO friendly, User friendly and responsive design website in the WordPress for your E commerce website, where user can buy and sell product both.
Requirement Analysis: Their are 3 types of user a) Buyer b) Seller and c) Admin
I proposed the core functionality based on your requirement as follows:
# Homepage: In the site buyer can see the product and guide in this section.
# Search Bar: Buyer can search their needed product by product category. We can do the auto complete suggestion in the search bar.
# Category: Buyer will able to view the existing category which the admin can add in the back end. Buyer can purchase their product from this section.
# Product Listing Page: Buyer can see the all product in the listing page.
# Product Details Page: In the listing page their is link of details where buyer click on the details they redirect to the details page.
# Add to cart: Buyer add the selected products in the shopping cart section
# Shopping Cart: When buyer add the selective items that items are go to the cart section.
# Checkout: Buyer can choose the payment gateway in this section.
# Payment gateway: Buyer make payment through PayPal, credit cards.
# Email Notifications: Buyer can receive the email notification after order be placed.
No More Space to write the whole proposal. Please come in chat
Thanks
Netdevbiz