This can be done using a single rollup field. Roll up the last name based on the {Linked to Contacts}, and use this aggregation formula:
IF(values, "Seller to " & ARRAYJOIN(ARRAYUNIQUE(values), " & "))

The ARRAYUNIQUE() function gets rid of duplicate last names, ARRAYJOIN() joins them with the ampersand, and the rest adds the prefix as long as there’s one incoming name.
This can be done using a single rollup field. Roll up the last name based on the {Linked to Contacts}, and use this aggregation formula:
IF(values, "Seller to " & ARRAYJOIN(ARRAYUNIQUE(values), " & "))

The ARRAYUNIQUE() function gets rid of duplicate last names, ARRAYJOIN() joins them with the ampersand, and the rest adds the prefix as long as there’s one incoming name.
Thank you so very much. This works perfectly.