You only need to have a business entity (which, to be fair, can be yourself, although liability concerns apply) once you're about to actually enter into a contract with someone.

Merely listing prices (without an associated automated payment form which could imply a contract was en force upon successful payment) doesn't actually force you to do business with anyone. It would however 1) deter requests for unpaid work by giving them an idea of what it would cost to get what they're looking for and 2) give you an idea of the demand for said services and could give you a warm lead which you can then choose to pursue formally by getting the necessary business structure, legal/business advice, etc.