Hi,

In this code the purchase order line only shows the products of corresponding vendor.

class ResPartner(models.Model):
    _inherit = 'purchase.order'

    is_vendor_products = fields.Boolean(string='Vendor Products')

class PurchaseOrderLine(models.Model):
    _inherit = 'purchase.order.line'

    @api.onchange('product_id')
    def vendor(self):
        list1 = []
        if self.order_id.is_vendor_products:
            for product in self.env['product.product'].search([]):
                for seller_ids in product.seller_ids:
                    if self.partner_id == seller_ids.name:
                        list1.append(product.id)
            return {'domain': {'product_id': [('id', 'in', list1)]}}

Regards

Override "search" method. This is recurrent question, try to search it first (even though I also a little bit sceptic about search feature in this forum)