var selectedIds = new[] { ObjectId.GenerateNewId() };
var result = postCollection
    .AsQueryable()
    .Where(p => selectedIds.Contains(p.Id) == false)
    .ToList();

Message: System.InvalidOperationException : Contains(value(MongoDB.Bson.ObjectId[])) is not supported. 
Stack Trace: 
PredicateTranslator.GetFieldExpression(Expression expression) 
PredicateTranslator.TranslateComparison(Expression variableExpression, ExpressionType operatorType, ConstantExpression constantExpression) 
PredicateTranslator.TranslateComparison(BinaryExpression binaryExpression)
PredicateTranslator.Translate(Expression node) 
PredicateTranslator.Translate(Expression node, IBsonSerializerRegistry serializerRegistry) 
QueryableTranslator.TranslateWhere(WhereExpression node) 
QueryableTranslator.Translate(Expression node) 
QueryableTranslator.TranslatePipeline(PipelineExpression node) 
QueryableTranslator.Translate(Expression node) 
QueryableTranslator.Translate(Expression node, IBsonSerializerRegistry serializerRegistry, ExpressionTranslationOptions translationOptions) 
MongoQueryProviderImpl`1.Translate(Expression expression) 
MongoQueryProviderImpl`1.Execute(Expression expression) 
MongoQueryableImpl`2.GetEnumerator() List`1.ctor(IEnumerable`1 collection) 
Enumerable.ToList[TSource](IEnumerable`1 source)