Tak nevím, mám tenhle testovací kód:

void Main()
{
	var list1 = new List<Dummy> { 
		new Dummy { Polozka = "a" },
		new Dummy { Polozka = "b" },
		new Dummy { Polozka = "c" },
		new Dummy { Polozka = "d" }
	};
	var list2 = new List<Dummy> {
		new Dummy { Polozka = "a" },
		new Dummy { Polozka = "b" },
		new Dummy { Polozka = "e" },
		new Dummy { Polozka = "f" }
	};
	
	var diff1 = list1.Where(t => !list2.Any(r => t.Polozka == r.Polozka));
	var diff2 = list2.Where(t => !list1.Any(r => t.Polozka == r.Polozka));
	
	Console.WriteLine(diff1);
	Console.WriteLine(diff2);
}

public class Dummy 
{
	public string Polozka { get; set; }
}

IEnumerable<Dummy> (2 items)
Polozka
c
d

IEnumerable<Dummy> (2 items)
Polozka
e
f