If I have a contract that has a list of sellers, how would I code a simple contract that if some condition applies, the contract in an automatic way will increase the reputation/rating of the sender ??
This is a very broad question, but one solution would be to simply assign a mapping
mapping(address => uint256) public rating that increments upon a good action and decrements upon a bad one.
For example, if the user successfully completes a payment, you can increment their rating mapping by one with
rating[user] += 1.