Finding people that sell a musical manifestation and finding people wanting that musical manifestation

1   PREFIX mo:   &lt;http://purl.org/ontology/mo/&gt; 2   PREFIX foaf: &lt;http://xmlns.com/foaf/0.1/&gt; 3   SELECT ?name_seller ?name_buyer 4   WHERE 5   { 6        { 7            ?seller a foaf: Person; 8                   foaf:name ?name_seller; 9                   mo:sell_item &lt;http://zitgist.com/music/record/a89e1d92-5381-4dab-ba51-733137d0e431&gt;. 10      } 11       UNION 12      { 13           ?seller a foaf: Person; 14                  foaf:name ?name_seller; 15                  mo:exchange_item &lt;http://zitgist.com/music/record/a89e1d92-5381-4dab-ba51-733137d0e431&gt;. 16      } 17       UNION 18      {     19           ?buyer a foaf: Person; 20                 foaf:name ?name_buyer; 21                 mo:want_item &lt;http://zitgist.com/music/record/a89e1d92-5381-4dab-ba51-733137d0e431&gt;. 22      } 23   }