Zahra:

It sounds like you want each buyer and seller to have a single account, such that they cannot manipulate their reputations by reviewing the same item or the same buyer/seller multiple times to inflate or deflate the average rating. An easy way to do this is to require each buyer and seller to prove to the system, when they create their account, that they control a credit card--and require them to disclose the credit card number to you in that proof. Then, don't allow the same credit card number to be used in more than one account, and don't allow a given account to rate the same item or buyer more than once per transaction.

If you don't want to use credit cards for your uniqueness, there is a way you can use the link secret to do the same thing. That's an advanced topic that I'll skip for now.

You might say, "Yes, but Alice and Bob might each have multiple credit cards, and might be able to set up multiple accounts that way." This is true, and if you truly want to force uniqueness per person, you can do something more draconian, like use a passport number instead of a credit card number as the source of uniqueness.

But before you go to that extremem, oonsider the following scenarios:

A) Alice wants to register multiple DIDs as a seller, because she has two businesses. One is for hand-made crafts that she does. The other is for used books that she sells. She would like her reputation as a seller of hand-made crafts to be different from her reputation as a seller of books.

B) Bob buys a lot of stuff online for himself, but he also buys a lot online for his company. He'd like to have two buyer accounts--one for personal buying, and one for company buying.

If you want to support either of these scenarios, then the credit card uniqueness is probably as far as you want to go.