Fixes in routes controllers

[tobisupreme]

In this PR:

Commit b1c33e5

• enforces the logic to check for existing business users

Commit 6fff2fc

• fixes the 404 error to the /users/:id route

Commit f220fa9

• improves the functionality to update seller (business user) details

Commit d37b8a3

• changes the type for the price field in the product validator from 'integer' to 'float'.