To prevent unwanted content in aggregation rows, use the isAutogeneratedRow helper from @mui/x-data-grid-premium inside renderCell or valueFormatter:

import { isAutogeneratedRow } from '@mui/x-data-grid-premium';

{
  field: 'marketplaceId',
  headerName: 'Marketplace',
  aggregable: false,
  renderCell: (params) => isAutogeneratedRow(params) ? null : params.value,
}