By default, a filter returns an array of values, even if you define the filter to only allow single values in the configuration.
To work your filter needs to be adjusted to reflect the array structure:

{%- if filter_values('marketplace') == ['LOOKUP'] -%}
 QUERY

note, if you are evaluating a query value as part of a where clause, you may need the format
and table.varchar_column = '{{filter_values('filter_field')[0] }}'