Let’s take that first. That can introduce a relative clause, regardless of whether the antecedent is human or not, when the relative clause defines the antecedent. Such clauses are variously called ‘defining, ‘restrictive’ or ‘integrated’. In the sentence ‘A seller that is interested will follow up the transaction’, ‘that is interested’ tells us what kind of seller will follow up the transaction, and so that is permissible. Who may be also be used.
Whether you use it to refer to ‘supplier’, 'buyer' and 'exporter' depends on how you regard them If you see them as some kind of abstraction, then it might be possible. If, as seems much more likely, you see them as predominantly human, then it won’t do. You then have to choose from he, she and they. There are obvious difficulties with the first two if you don’t know if the supplier is male or female. In many cases, they can refer to a singular antecedent, but it is a little awkward in your examples. The best solution might be to make supplier, buyer and exporter plural, so that you get the sentences:
The suppliers will respond after they have received a communication.
The buyers themselves may take on the responsibility.
The exporters must disclose their supply capacity.