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It depends on how you are setting those forms for your sellers.
Are you a website where sellers can post their product and buyers can search and pay for them?
Or you are setting up websites for specific sellers?
If you want to create a contact form for each seller and embed it to their website, that is possible by creating separate forms for each one. Then, setup their email notifications to be sent to the seller's email address.
When the buyer fills out the contact form, the email notification will be sent to the email address of the seller, the seller can reply in the email directly and send it to the sender if you setup the "Reply-To" field directly in the email settings.
You can also check our templates for the sample contact forms: https://www.jotform.com/form-templates/category/contact-form
If I misunderstood your concern, please do provide us more details of how your system works.